{"cve":"CVE-2021-46933","nvd":{"published":"2024-02-27T10:15:07.807","modified":"2026-08-04T10:16:37.357","status":"Modified","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nusb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.\n\nffs_data_clear is indirectly called from both ffs_fs_kill_sb and\nffs_ep0_release, so it ends up being called twice when userland closes ep0\nand then unmounts f_fs.\nIf userland provided an eventfd along with function's USB descriptors, it\nends up calling eventfd_ctx_put as many times, causing a refcount\nunderflow.\nNULL-ify ffs_eventfd to prevent these extraneous eventfd_ctx_put calls.\n\nAlso, set epfiles to NULL right after de-allocating it, for readability.\n\nFor completeness, ffs_data_clear actually ends up being called thrice, the\nlast call being before the whole ffs structure gets freed, so when this\nspecific sequence happens there is a second underflow happening (but not\nbeing reported):\n\n/sys/kernel/debug/tracing# modprobe usb_f_fs\n/sys/kernel/debug/tracing# echo ffs_data_clear > set_ftrace_filter\n/sys/kernel/debug/tracing# echo function > current_tracer\n/sys/kernel/debug/tracing# echo 1 > tracing_on\n(setup gadget, run and kill function userland process, teardown gadget)\n/sys/kernel/debug/tracing# echo 0 > tracing_on\n/sys/kernel/debug/tracing# cat trace\n smartcard-openp-436     [000] .....  1946.208786: ffs_data_clear <-ffs_data_closed\n smartcard-openp-431     [000] .....  1946.279147: ffs_data_clear <-ffs_data_closed\n smartcard-openp-431     [000] .n...  1946.905512: ffs_data_clear <-ffs_data_put\n\nWarning output corresponding to above trace:\n[ 1946.284139] WARNING: CPU: 0 PID: 431 at lib/refcount.c:28 refcount_warn_saturate+0x110/0x15c\n[ 1946.293094] refcount_t: underflow; use-after-free.\n[ 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)\n[ 1946.399633] CPU: 0 PID: 431 Comm: smartcard-openp Tainted: G         C OE     5.15.0-1-rpi #1  Debian 5.15.3-1\n[ 1946.417950] Hardware name: BCM2835\n[ 1946.425442] Backtrace:\n[ 1946.432048] [<c08d60a0>] (dump_backtrace) from [<c08d62ec>] (show_stack+0x20/0x24)\n[ 1946.448226]  r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c\n[ 1946.458412] [<c08d62cc>] (show_stack) from [<c08d9ae0>] (dump_stack+0x28/0x30)\n[ 1946.470380] [<c08d9ab8>] (dump_stack) from [<c0123500>] (__warn+0xe8/0x154)\n[ 1946.482067]  r5:c04a948c r4:c0a71dc8\n[ 1946.490184] [<c0123418>] (__warn) from [<c08d6948>] (warn_slowpath_fmt+0xa0/0xe4)\n[ 1946.506758]  r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04\n[ 1946.517070] [<c08d68ac>] (warn_slowpath_fmt) from [<c04a948c>] (refcount_warn_saturate+0x110/0x15c)\n[ 1946.535309]  r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0\n[ 1946.546708] [<c04a937c>] (refcount_warn_saturate) from [<c0380134>] (eventfd_ctx_put+0x48/0x74)\n[ 1946.564476] [<c03800ec>] (eventfd_ctx_put) from [<bf5464e8>] (ffs_data_clear+0xd0/0x118 [usb_f_fs])\n[ 1946.582664]  r5:c3b84c00 r4:c2695b00\n[ 1946.590668] [<bf546418>] (ffs_data_clear [usb_f_fs]) from [<bf547cc0>] (ffs_data_closed+0x9c/0x150 [usb_f_fs])\n[ 1946.609608]  r5:bf54d014 r4:c2695b00\n[ 1946.617522] [<bf547c24>] (ffs_data_closed [usb_f_fs]) from [<bf547da0>] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs])\n[ 1946.636217]  r7:c0dfcb\n---truncated---","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","score":5.5},"cwes":["CWE-416","CWE-476"],"references":["https://git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645","https://git.kernel.org/stable/c/240fc586e83d645912accce081a48aa63a45f6ee","https://git.kernel.org/stable/c/33f6a0cbb7772146e1c11f38028fffbfed14728b","https://git.kernel.org/stable/c/52500239e3f2d6fc77b6f58632a9fb98fe74ac09","https://git.kernel.org/stable/c/b1e0887379422975f237d43d8839b751a6bcf154"],"products":["linux:linux_kernel"],"source":"nvd","synced_at":"2026-10-19T21:45:59.823092188+00:00"},"epss":{"cve":"CVE-2021-46933","score":0.00244,"percentile":0.15684,"date":"2026-10-31"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}