{ "schemaVersion": 1, "attribution": [ { "source": "CISA Known Exploited Vulnerabilities", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog", "licence": "US government work" }, { "source": "GitHub Advisory Database", "url": "https://github.com/advisories", "licence": "CC BY 4.0" }, { "source": "Have I Been Pwned", "url": "https://haveibeenpwned.com/", "licence": "CC BY 4.0" }, { "source": "CVE Program (CVE records)", "url": "https://www.cve.org/Legal/TermsOfUse", "licence": "CVE Terms of Use", "notice": "Copyright © 1999-2026, The MITRE Corporation. CVE is a trademark and the CVE logo is a registered trademark of The MITRE Corporation.", "terms": "CVE Usage: MITRE hereby grants you a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare derivative works of, publicly display, publicly perform, sublicense, and distribute Common Vulnerabilities and Exposures (CVE™). Any copy you make for such purposes is authorized provided that you reproduce MITRE's copyright designation and this license in any such copy." }, { "source": "FIRST EPSS", "url": "https://www.first.org/epss/", "licence": "free to use; FIRST requests attribution" } ], "record": { "id": "CVE-2021-46933", "sourceURL": "https://www.cve.org/CVERecord?id=CVE-2021-46933", "idYear": 2021, "status": "PUBLISHED", "publishedAt": "2024-02-27T09:44:00.758Z", "updatedAt": "2026-08-05T08:45:09.548Z", "importedAt": "2026-10-24T01:56:36.661Z", "snapshot": { "id": "cve_2026-10-24_midnight_baseline", "at": "2026-10-24T00:00:00.000Z", "source": "https://github.com/CVEProject/cvelistV5/releases/tag/cve_2026-10-24_0100Z" }, "facts": { "title": "usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.", "summary": "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] [] (dump_backtrace) from [] (show_stack+0x20/0x24)\n[ 1946.448226] r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c\n[ 1946.458412] [] (show_stack) from [] (dump_stack+0x28/0x30)\n[ 1946.470380] [] (dump_stack) from [] (__warn+0xe8/0x154)\n[ 1946.482067] r5:c04a948c r4:c0a71dc8\n[ 1946.490184] [] (__warn) from [] (warn_slowpath_fmt+0xa0/0xe4)\n[ 1946.506758] r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04\n[ 1946.517070] [] (warn_slowpath_fmt) from [] (refcount_warn_saturate+0x110/0x15c)\n[ 1946.535309] r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0\n[ 1946.546708] [] (refcount_warn_saturate) from [] (eventfd_ctx_put+0x48/0x74)\n[ 1946.564476] [] (eventfd_ctx_put) from [] (ffs_data_clear+0xd0/0x118 [usb_f_fs])\n[ 1946.582664] r5:c3b84c00 r4:c2695b00\n[ 1946.590668] [] (ffs_data_clear [usb_f_fs]) from [] (ffs_data_closed+0x9c/0x150 [usb_f_fs])\n[ 1946.609608] r5:bf54d014 r4:c2695b00\n[ 1946.617522] [] (ffs_data_closed [usb_f_fs]) from [] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs])\n[ 1946.636217] r7:c0dfcb\n---truncated---", "affected": [ { "vendor": "Linux", "product": "Linux", "defaultStatus": "unaffected", "versions": [ { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "f976dd7011150244a7ba820f2c331e9fb253befa" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "cc8c8028c21b2a3842a1e98e99e55028df275919" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "52500239e3f2d6fc77b6f58632a9fb98fe74ac09" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "33f6a0cbb7772146e1c11f38028fffbfed14728b" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "240fc586e83d645912accce081a48aa63a45f6ee" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "1c4ace3e6b8575745c50dca9e76e0021e697d645" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "ebef2aa29f370b5096c16020c104e393192ef684" }, { "version": "5e33f6fdf735cda1d4580fe6f1878da05718fe73", "status": "affected", "versionType": "git", "lessThan": "b1e0887379422975f237d43d8839b751a6bcf154" } ] }, { "vendor": "Linux", "product": "Linux", "defaultStatus": "affected", "versions": [ { "version": "4.0", "status": "affected" }, { "version": "0", "status": "unaffected", "versionType": "semver", "lessThan": "4.0" }, { "version": "4.4.298", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "4.4.*" }, { "version": "4.9.296", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "4.9.*" }, { "version": "4.14.261", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "4.14.*" }, { "version": "4.19.224", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "4.19.*" }, { "version": "5.4.170", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "5.4.*" }, { "version": "5.10.90", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "5.10.*" }, { "version": "5.15.13", "status": "unaffected", "versionType": "semver", "lessThanOrEqual": "5.15.*" }, { "version": "5.16", "status": "unaffected", "versionType": "original_commit_for_fix", "lessThanOrEqual": "*" } ] } ] } } }