CVE-2026-74635 (GCVE-0-2026-74635)
Vulnerability from cvelistv5
Published
2026-08-22 15:32
Modified
2026-08-25 05:40
Severity ?
VLAI Severity ?
EPSS score ?
Summary
In the Linux kernel, the following vulnerability has been resolved:
fbdev: bitblit: bound-check glyph index in bit_cursor()
bit_cursor() fetches the glyph under the cursor with
c = scr_readw(vc_pos);
src = vc_font.data + ((c & charmask) * w * height);
where charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer
value comes directly from scr_readw() and may be larger than the current
font's glyph count.
Syzkaller triggers this via vcs_write(). The Call Trace shows
vcs_write() in vc_screen.c writing an arbitrary 16-bit value with
writev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via
vcs_scr_writew() without checking charcount. The stored value is later
read in bit_cursor() in bitblit.c.
When the font is changed from a font with 512 glyphs to a font with
256 glyphs, the screen buffer can retain characters with the high
bit set from the previous mode, which could also produce the same
out-of-bounds access.
BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70
Read of size 16 at addr ffff800086c57970
Call Trace:
soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70
bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365
fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427
hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883
update_region+0x100/0x18c drivers/tty/vt/vt.c:669
vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685
bit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph
index to vc_font.charcount. Apply the same clamp in bit_cursor() after
extracting the attribute and masking, before indexing fontdata.
The fix completes the bounds checking started in commit 18c4ef4e765a
("fbdev: bitblit: bound-check glyph index in bit_putcs*"), which missed
the cursor path.
This change should be safe because the clamp reuses the existing
contract from fbcon: charcount is maintained under console_lock in
con_font_set() and fbcon_font_set(), and hi_font_mask is cleared when
switching from 512 to 256 glyphs. When stale screen data with high bits
remains after a font switch, or when vcs_write() stores an arbitrary
value, clamping the index to 0 prevents the out-of-bounds read without
changing cursor semantics — the same fallback bit_putcs uses.
References
Impacted products
| Vendor | Product | Version | ||
|---|---|---|---|---|
| Linux | Linux |
Version: 0998a6cb232674408a03e8561dc15aa266b2f53b Version: db5c9a162d2f42bcc842b76b3d935dcc050a0eec Version: c12003bf91fdff381c55ef54fef3e961a5af2545 Version: 9ba1a7802ca9a2590cef95b253e6526f4364477f Version: 901f44227072be60812fe8083e83e1533c04eed1 Version: 18c4ef4e765a798b47980555ed665d78b71aeadf Version: 18c4ef4e765a798b47980555ed665d78b71aeadf Version: 18c4ef4e765a798b47980555ed665d78b71aeadf Version: a10cede006f9614b465cf25609a8753efbfd45cc Version: efaf89a75a29b2d179bf4fe63ca62852e93ad620 Version: 5.10.247 ≤ Version: 5.15.197 ≤ Version: 6.1.159 ≤ Version: 6.6.117 ≤ Version: 6.12.58 ≤ Version: 5.4.302 ≤ Version: 6.17.8 ≤ |
||
{
"containers": {
"cna": {
"affected": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"drivers/video/fbdev/core/bitblit.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"lessThan": "250159ace2dc53c1bdad267aa8da51b638748700",
"status": "affected",
"version": "0998a6cb232674408a03e8561dc15aa266b2f53b",
"versionType": "git"
},
{
"lessThan": "46336f476484f36145e5117e72d7b590f47433ee",
"status": "affected",
"version": "db5c9a162d2f42bcc842b76b3d935dcc050a0eec",
"versionType": "git"
},
{
"lessThan": "bf750cfeacf4e47ac72dadc7f05839696efb8576",
"status": "affected",
"version": "c12003bf91fdff381c55ef54fef3e961a5af2545",
"versionType": "git"
},
{
"lessThan": "94134d70abf9273b70499d97d0adc9185ef21091",
"status": "affected",
"version": "9ba1a7802ca9a2590cef95b253e6526f4364477f",
"versionType": "git"
},
{
"lessThan": "c1e7351767dd30fc574395c82121e4c67b882da3",
"status": "affected",
"version": "901f44227072be60812fe8083e83e1533c04eed1",
"versionType": "git"
},
{
"lessThan": "bc9db0d879c655d5dfd8add32fd60f13e65d132c",
"status": "affected",
"version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
"versionType": "git"
},
{
"lessThan": "9ea879862e66e354e616028c31b39aa3eb6d35d8",
"status": "affected",
"version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
"versionType": "git"
},
{
"lessThan": "e033cbf3975a8465f879ebd5989dc35b04423a4d",
"status": "affected",
"version": "18c4ef4e765a798b47980555ed665d78b71aeadf",
"versionType": "git"
},
{
"status": "affected",
"version": "a10cede006f9614b465cf25609a8753efbfd45cc",
"versionType": "git"
},
{
"status": "affected",
"version": "efaf89a75a29b2d179bf4fe63ca62852e93ad620",
"versionType": "git"
},
{
"lessThan": "5.10.265",
"status": "affected",
"version": "5.10.247",
"versionType": "semver"
},
{
"lessThan": "5.15.216",
"status": "affected",
"version": "5.15.197",
"versionType": "semver"
},
{
"lessThan": "6.1.183",
"status": "affected",
"version": "6.1.159",
"versionType": "semver"
},
{
"lessThan": "6.6.152",
"status": "affected",
"version": "6.6.117",
"versionType": "semver"
},
{
"lessThan": "6.12.104",
"status": "affected",
"version": "6.12.58",
"versionType": "semver"
},
{
"lessThan": "5.5",
"status": "affected",
"version": "5.4.302",
"versionType": "semver"
},
{
"lessThan": "6.18",
"status": "affected",
"version": "6.17.8",
"versionType": "semver"
}
]
},
{
"defaultStatus": "affected",
"product": "Linux",
"programFiles": [
"drivers/video/fbdev/core/bitblit.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"status": "affected",
"version": "6.18"
},
{
"lessThan": "6.18",
"status": "unaffected",
"version": "0",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.10.*",
"status": "unaffected",
"version": "5.10.265",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.15.*",
"status": "unaffected",
"version": "5.15.216",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.1.*",
"status": "unaffected",
"version": "6.1.183",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.6.*",
"status": "unaffected",
"version": "6.6.152",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.12.*",
"status": "unaffected",
"version": "6.12.104",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.18.*",
"status": "unaffected",
"version": "6.18.45",
"versionType": "semver"
},
{
"lessThanOrEqual": "7.1.*",
"status": "unaffected",
"version": "7.1.9",
"versionType": "semver"
},
{
"lessThanOrEqual": "*",
"status": "unaffected",
"version": "7.2",
"versionType": "original_commit_for_fix"
}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.10.265",
"versionStartIncluding": "5.10.247",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.15.216",
"versionStartIncluding": "5.15.197",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.1.183",
"versionStartIncluding": "6.1.159",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.6.152",
"versionStartIncluding": "6.6.117",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.12.104",
"versionStartIncluding": "6.12.58",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.18.45",
"versionStartIncluding": "6.18",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "7.1.9",
"versionStartIncluding": "6.18",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "7.2",
"versionStartIncluding": "6.18",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionStartIncluding": "5.4.302",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionStartIncluding": "6.17.8",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfbdev: bitblit: bound-check glyph index in bit_cursor()\n\nbit_cursor() fetches the glyph under the cursor with\n\n\tc = scr_readw(vc_pos);\n\tsrc = vc_font.data + ((c \u0026 charmask) * w * height);\n\nwhere charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer\nvalue comes directly from scr_readw() and may be larger than the current\nfont\u0027s glyph count.\n\nSyzkaller triggers this via vcs_write(). The Call Trace shows\nvcs_write() in vc_screen.c writing an arbitrary 16-bit value with\nwritev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via\nvcs_scr_writew() without checking charcount. The stored value is later\nread in bit_cursor() in bitblit.c.\n\nWhen the font is changed from a font with 512 glyphs to a font with\n256 glyphs, the screen buffer can retain characters with the high\nbit set from the previous mode, which could also produce the same\nout-of-bounds access.\n\n BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n Read of size 16 at addr ffff800086c57970\n\n Call Trace:\n soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365\n fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427\n hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883\n update_region+0x100/0x18c drivers/tty/vt/vt.c:669\n vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685\n\nbit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph\nindex to vc_font.charcount. Apply the same clamp in bit_cursor() after\nextracting the attribute and masking, before indexing fontdata.\n\nThe fix completes the bounds checking started in commit 18c4ef4e765a\n(\"fbdev: bitblit: bound-check glyph index in bit_putcs*\"), which missed\nthe cursor path.\n\nThis change should be safe because the clamp reuses the existing\ncontract from fbcon: charcount is maintained under console_lock in\ncon_font_set() and fbcon_font_set(), and hi_font_mask is cleared when\nswitching from 512 to 256 glyphs. When stale screen data with high bits\nremains after a font switch, or when vcs_write() stores an arbitrary\nvalue, clamping the index to 0 prevents the out-of-bounds read without\nchanging cursor semantics \u2014 the same fallback bit_putcs uses."
}
],
"metrics": [
{
"cvssV3_1": {
"baseScore": 7.8,
"baseSeverity": "HIGH",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"version": "3.1"
},
"scenarios": [
{
"lang": "en",
"value": "AV:L - Exploitation requires local write() to /dev/vcsa (or ioctl on an owned virtual console) to poison the VT screen buffer; the bug is in fbcon cursor rendering, not in any network-facing protocol handler.\nAC:L - Syzkaller reproduces the fault reliably by writing arbitrary screen-buffer values to /dev/vcsa; the attacker fully controls the glyph index and can repeat the trigger on every console refresh without races or victim-specific timing.\nPR:L - Triggering needs only a local account with write access to /dev/vcsa* (typically membership in the tty group) or a session owning the target console for KDFONTOP/font-switch paths; no CAP_SYS_ADMIN or init-namespace root is required.\nUI:N - Once the attacker has the needed local device access, exploitation is fully self-contained and does not require any additional action from another user or administrator.\nS:U - Impact is confined to kernel framebuffer-console memory within the same kernel security domain; it does not cross VM, container, or IOMMU boundaries to affect a separate authority.\nC:H - An out-of-bounds read past the font glyph table copies adjacent kernel memory (global or heap font data) into cursor buffers and framebuffer blit paths, disclosing more than a few bytes per trigger.\nI:H - The out-of-bounds glyph pointer feeds update_attr() and soft_cursor() memcpy paths that copy attacker-influenced kernel memory into kmalloc cursor buffers and framebuffer state, constituting kernel memory corruption with integrity impact.\nA:H - KASAN reports a kernel BUG on the out-of-bounds read in soft_cursor(), and repeated malicious /dev/vcsa writes force console refresh through hide_cursor(), enabling kernel oops/panic and console denial of service."
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-25T05:40:59.420Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700"
},
{
"url": "https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee"
},
{
"url": "https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576"
},
{
"url": "https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091"
},
{
"url": "https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3"
},
{
"url": "https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c"
},
{
"url": "https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8"
},
{
"url": "https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d"
}
],
"title": "fbdev: bitblit: bound-check glyph index in bit_cursor()",
"x_generator": {
"engine": "bippy-1.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2026-74635",
"datePublished": "2026-08-22T15:32:15.280Z",
"dateReserved": "2026-08-15T05:44:03.922Z",
"dateUpdated": "2026-08-25T05:40:59.420Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
Loading…
Loading…
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.
Loading…
Loading…