NVD / CVE Program· CNA Linux

CVE-2021-46933: usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.

MediumCVSS 5.5 · v3.1—No exploit Fix available

Noroxi assessment

Priority: Monitor (action score 22/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.24% (FIRST EPSS).
Someone with local access to the system can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
Fixed version: Linux Linux 1c4ace3e6b8575745c50dca9e76e0021e697d645, 240fc586e83d645912accce081a48aa63a45f6ee, 33f6a0cbb7772146e1c11f38028fffbfed14728b.
Severity is disputed: the assigning authority (CNA) scores it 7.8, NVD 5.5.
Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to Linux Linux 1c4ace3e6b8575745c50dca9e76e0021e697d645, 240fc586e83d645912accce081a48aa63a45f6ee, 33f6a0cbb7772146e1c11f38028fffbfed14728b or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear. ffs_data_clear is indirectly called from both ffs_fs_kill_sb and ffs_ep0_release, so it ends up being called twice when userland closes ep0 and then unmounts f_fs. If userland provided an eventfd along with function's USB descriptors, it ends up calling eventfd_ctx_put as many times, causing a refcount underflow. NULL-ify ffs_eventfd to prevent these extraneous eventfd_ctx_put calls. Also, set epfiles to NULL right after de-allocating it, for readability. For completeness, ffs_data_clear actually ends up being called thrice, the last call being before the whole ffs structure gets freed, so when this specific sequence happens there is a second underflow happening (but not being reported): /sys/kernel/debug/tracing# modprobe usb_f_fs /sys/kernel/debug/tracing# echo ffs_data_clear > set_ftrace_filter /sys/kernel/debug/tracing# echo function > current_tracer /sys/kernel/debug/tracing# echo 1 > tracing_on (setup gadget, run and kill function userland process, teardown gadget) /sys/kernel/debug/tracing# echo 0 > tracing_on /sys/kernel/debug/tracing# cat trace smartcard-openp-436 [000] ..... 1946.208786: ffs_data_clear <-ffs_data_closed smartcard-openp-431 [000] ..... 1946.279147: ffs_data_clear <-ffs_data_closed smartcard-openp-431 [000] .n... 1946.905512: ffs_data_clear <-ffs_data_put Warning output corresponding to above trace: [ 1946.284139] WARNING: CPU: 0 PID: 431 at lib/refcount.c:28 refcount_warn_saturate+0x110/0x15c [ 1946.293094] refcount_t: underflow; use-after-free. [ 1946.298164] Modules linked in: usb_f_ncm(E) u_ether(E) usb_f_fs(E) hci_uart(E) btqca(E) btrtl(E) btbcm(E) btintel(E) bluetooth(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) bcm2835_v4l2(CE) bcm2835_mmal_vchiq(CE) videobuf2_vmalloc(E) videobuf2_memops(E) sha512_generic(E) videobuf2_v4l2(E) sha512_arm(E) videobuf2_common(E) videodev(E) cpufreq_dt(E) snd_bcm2835(CE) brcmfmac(E) mc(E) vc4(E) ctr(E) brcmutil(E) snd_soc_core(E) snd_pcm_dmaengine(E) drbg(E) snd_pcm(E) snd_timer(E) snd(E) soundcore(E) drm_kms_helper(E) cec(E) ansi_cprng(E) rc_core(E) syscopyarea(E) raspberrypi_cpufreq(E) sysfillrect(E) sysimgblt(E) cfg80211(E) max17040_battery(OE) raspberrypi_hwmon(E) fb_sys_fops(E) regmap_i2c(E) ecdh_generic(E) rfkill(E) ecc(E) bcm2835_rng(E) rng_core(E) vchiq(CE) leds_gpio(E) libcomposite(E) fuse(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) crc32c_generic(E) sdhci_iproc(E) sdhci_pltfm(E) sdhci(E) [ 1946.399633] CPU: 0 PID: 431 Comm: smartcard-openp Tainted: G C OE 5.15.0-1-rpi #1 Debian 5.15.3-1 [ 1946.417950] Hardware name: BCM2835 [ 1946.425442] Backtrace: [ 1946.432048] [<c08d60a0>] (dump_backtrace) from [<c08d62ec>] (show_stack+0x20/0x24) [ 1946.448226] r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c [ 1946.458412] [<c08d62cc>] (show_stack) from [<c08d9ae0>] (dump_stack+0x28/0x30) [ 1946.470380] [<c08d9ab8>] (dump_stack) from [<c0123500>] (__warn+0xe8/0x154) [ 1946.482067] r5:c04a948c r4:c0a71dc8 [ 1946.490184] [<c0123418>] (__warn) from [<c08d6948>] (warn_slowpath_fmt+0xa0/0xe4) [ 1946.506758] r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04 [ 1946.517070] [<c08d68ac>] (warn_slowpath_fmt) from [<c04a948c>] (refcount_warn_saturate+0x110/0x15c) [ 1946.535309] r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0 [ 1946.546708] [<c04a937c>] (refcount_warn_saturate) from [<c0380134>] (eventfd_ctx_put+0x48/0x74) [ 1946.564476] [<c03800ec>] (eventfd_ctx_put) from [<bf5464e8>] (ffs_data_clear+0xd0/0x118 [usb_f_fs]) [ 1946.582664] r5:c3b84c00 r4:c2695b00 [ 1946.590668] [<bf546418>] (ffs_data_clear [usb_f_fs]) from [<bf547cc0>] (ffs_data_closed+0x9c/0x150 [usb_f_fs]) [ 1946.609608] r5:bf54d014 r4:c2695b00 [ 1946.617522] [<bf547c24>] (ffs_data_closed [usb_f_fs]) from [<bf547da0>] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs]) [ 1946.636217] r7:c0dfcb ---truncated---

Published: Feb 27, 2024
Updated: Aug 5, 2026
EPSS: 0.2% · 15th percentile
CWE: CWE-416

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 22 / 40 · 5.5 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.2%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: 5.5
CNA · Linux: 7.8; 2.3 point gap

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
linux	linux kernel	cpe:2.3:o:linux:linux_kernel

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Linux). Independent of NVD's CPE analysis and usually ahead of it.

Linux Linux
- 4.0affected
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before f976dd7011150244a7ba820f2c331e9fb253befaaffected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before cc8c8028c21b2a3842a1e98e99e55028df275919affected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before 52500239e3f2d6fc77b6f58632a9fb98fe74ac09affected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before 33f6a0cbb7772146e1c11f38028fffbfed14728baffected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before 240fc586e83d645912accce081a48aa63a45f6eeaffected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before 1c4ace3e6b8575745c50dca9e76e0021e697d645affected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before ebef2aa29f370b5096c16020c104e393192ef684affected · git
- 5e33f6fdf735cda1d4580fe6f1878da05718fe73 and later · before b1e0887379422975f237d43d8839b751a6bcf154affected · git
- before 4.0not affected · semver

Affected versions · NVD (7)

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

linux linux kernel4.0.0 and later · before 4.4.298
linux linux kernel4.5.0 and later · before 4.9.296
linux linux kernel4.10.0 and later · before 4.14.261
linux linux kernel4.15.0 and later · before 4.19.224
linux linux kernel4.20.0 and later · before 5.4.170
linux linux kernel5.5.0 and later · before 5.10.90
linux linux kernel5.11.0 and later · before 5.15.13

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:12	linux	before 5.15.15-1	5.15.15-1
openSUSE:Leap 15.5	dtb-aarch64	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-64kb	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-azure	before 5.14.21-150500.33.48.1	5.14.21-150500.33.48.1
openSUSE:Leap 15.5	kernel-debug	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-default	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-default-base	before 5.14.21-150500.55.59.1.150500.6.25.7	5.14.21-150500.55.59.1.150500.6.25.7
openSUSE:Leap 15.5	kernel-docs	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-kvmsmall	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-obs-build	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-obs-qa	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-rt	before 5.14.21-150500.13.47.1	5.14.21-150500.13.47.1
openSUSE:Leap 15.5	kernel-rt_debug	before 5.14.21-150500.13.47.1	5.14.21-150500.13.47.1
openSUSE:Leap 15.5	kernel-source	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-source-azure	before 5.14.21-150500.33.48.1	5.14.21-150500.33.48.1
openSUSE:Leap 15.5	kernel-source-rt	before 5.14.21-150500.13.47.1	5.14.21-150500.13.47.1
openSUSE:Leap 15.5	kernel-syms	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap 15.5	kernel-syms-azure	before 5.14.21-150500.33.48.1	5.14.21-150500.33.48.1
openSUSE:Leap 15.5	kernel-syms-rt	before 5.14.21-150500.13.47.1	5.14.21-150500.13.47.1
openSUSE:Leap 15.5	kernel-zfcpdump	before 5.14.21-150500.55.59.1	5.14.21-150500.55.59.1
openSUSE:Leap Micro 5.3	kernel-default	before 5.14.21-150400.24.116.1	5.14.21-150400.24.116.1
openSUSE:Leap Micro 5.3	kernel-default-base	before 5.14.21-150400.24.116.1.150400.24.54.5	5.14.21-150400.24.116.1.150400.24.54.5
openSUSE:Leap Micro 5.3	kernel-rt	before 5.14.21-150400.15.76.1	5.14.21-150400.15.76.1
SUSE:Linux Enterprise High Availability Extension 12 SP5	kernel-default	before 4.12.14-122.219.1	4.12.14-122.219.1

+52

Same product

linux: all records

Other highest-scoring records for the same primary product.

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Published before and after this one in the same product

← EarlierCVE-2021-46931Feb 27, 2024 · net/mlx5e: Wrap the tx reporter dump callback to extract the sq
22Monitor
← EarlierCVE-2021-46932Feb 27, 2024 · Input: appletouch - initialize work before device registration
22Monitor
Later →CVE-2021-46934Feb 27, 2024 · i2c: validate user data in compat ioctl
13Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
Linux Linux	1c4ace3e6b8575745c50dca9e76e0021e697d645	Vendor (CNA)
Linux Linux	240fc586e83d645912accce081a48aa63a45f6ee	Vendor (CNA)
Linux Linux	33f6a0cbb7772146e1c11f38028fffbfed14728b	Vendor (CNA)
Linux Linux	52500239e3f2d6fc77b6f58632a9fb98fe74ac09	Vendor (CNA)
Linux Linux	b1e0887379422975f237d43d8839b751a6bcf154	Vendor (CNA)
Linux Linux	cc8c8028c21b2a3842a1e98e99e55028df275919	Vendor (CNA)
Linux Linux	ebef2aa29f370b5096c16020c104e393192ef684	Vendor (CNA)
Linux Linux	f976dd7011150244a7ba820f2c331e9fb253befa	Vendor (CNA)
debian:linux	5.15.15-1 · Debian:12	Package registry (OSV)
opensuse:dtb-aarch64	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-64kb	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-debug	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-default	5.14.21-150400.24.116.1 · openSUSE:Leap Micro 5.3	Package registry (OSV)
opensuse:kernel-default-base	5.14.21-150400.24.116.1.150400.24.54.5 · openSUSE:Leap Micro 5.3	Package registry (OSV)
opensuse:kernel-docs	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-kvmsmall	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-obs-build	5.14.21-150500.55.59.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-rt	5.14.21-150400.15.76.1 · openSUSE:Leap Micro 5.3	Package registry (OSV)
opensuse:kernel-rt_debug	5.14.21-150500.13.47.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-source-rt	5.14.21-150500.13.47.1 · openSUSE:Leap 15.5	Package registry (OSV)
opensuse:kernel-syms-rt	5.14.21-150500.13.47.1 · openSUSE:Leap 15.5	Package registry (OSV)
suse:kernel-64kb	5.14.21-150400.24.116.1 · SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS	Package registry (OSV)
suse:kernel-default	5.14.21-150400.24.116.1 · SUSE:Linux Enterprise Micro 5.3	Package registry (OSV)
suse:kernel-default-base	5.14.21-150400.24.116.1.150400.24.54.5 · SUSE:Linux Enterprise Micro 5.3	Package registry (OSV)
suse:kernel-docs	5.14.21-150400.24.116.1 · SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS	Package registry (OSV)
suse:kernel-livepatch-SLE15-SP4_Update_25	1-150400.9.5.1 · SUSE:Linux Enterprise Live Patching 15 SP4	Package registry (OSV)
suse:kernel-livepatch-SLE15-SP5-RT_Update_13	1-150500.11.3.1 · SUSE:Linux Enterprise Live Patching 15 SP5	Package registry (OSV)
suse:kernel-obs-build	5.14.21-150400.24.116.1 · SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS	Package registry (OSV)
suse:kernel-rt	5.14.21-150400.15.76.1 · SUSE:Linux Enterprise Micro 5.3	Package registry (OSV)
suse:kernel-source	5.14.21-150400.24.116.1 · SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

TR-24-0138 · Feb 28, 2024(Linux Kernel Güncellemesi)

All national notices →

Not on record for this CVE: EPSS history · patch commit or PR link · researcher credit · variant or chain candidate · public bounty program.

Technical details

Attack conditions

Someone with local access to the system can trigger it.
A low-privileged account is enough.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: none
Integrity: none
Availability: high · the service can be disrupted

Attack vector: Local
Attack complexity: Low
Privileges required: Low
User interaction: None
Scope: Unchanged
Confidentiality impact: None
Integrity impact: None
Availability impact: High

Weakness class (CWE)

CWE-416 · Use After Free

CVSS vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

nvd-primary

Root cause

After a memory region is freed, another reference pointing to it is still used. By then, the same region may have been allocated for different data.

The bug in code

The whole CWE-416 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1free(dev->buf);
2/* ... another thread is still running ... */
3process(dev->buf);

Fixed

1lock(&dev->lock);
2free(dev->buf);
3dev->buf = NULL;
4unlock(&dev->lock);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log (1)

Fix✗ → ✓Sep 28, 2026, 10:09 PM UTC

For records you follow, these changes also arrive as notifications. →

References

git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645

Show all references (7 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records