fbdev: bitblit: bound-check glyph index in bit_cursor()
Summary
| CVE | CVE-2026-74635 |
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| State | PUBLISHED |
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| Assigner | Linux |
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| Source Priority | CVE Program / NVD first with legacy fallback |
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| Published | 2026-08-22 16:16:36 UTC |
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| Updated | 2026-08-22 16:16:36 UTC |
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| Description | 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. |
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Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|
| CNA |
Linux |
Linux |
affected 0998a6cb232674408a03e8561dc15aa266b2f53b 250159ace2dc53c1bdad267aa8da51b638748700 git |
Not specified |
| CNA |
Linux |
Linux |
affected db5c9a162d2f42bcc842b76b3d935dcc050a0eec 46336f476484f36145e5117e72d7b590f47433ee git |
Not specified |
| CNA |
Linux |
Linux |
affected c12003bf91fdff381c55ef54fef3e961a5af2545 bf750cfeacf4e47ac72dadc7f05839696efb8576 git |
Not specified |
| CNA |
Linux |
Linux |
affected 9ba1a7802ca9a2590cef95b253e6526f4364477f 94134d70abf9273b70499d97d0adc9185ef21091 git |
Not specified |
| CNA |
Linux |
Linux |
affected 901f44227072be60812fe8083e83e1533c04eed1 c1e7351767dd30fc574395c82121e4c67b882da3 git |
Not specified |
| CNA |
Linux |
Linux |
affected 18c4ef4e765a798b47980555ed665d78b71aeadf bc9db0d879c655d5dfd8add32fd60f13e65d132c git |
Not specified |
| CNA |
Linux |
Linux |
affected 18c4ef4e765a798b47980555ed665d78b71aeadf 9ea879862e66e354e616028c31b39aa3eb6d35d8 git |
Not specified |
| CNA |
Linux |
Linux |
affected 18c4ef4e765a798b47980555ed665d78b71aeadf e033cbf3975a8465f879ebd5989dc35b04423a4d git |
Not specified |
| CNA |
Linux |
Linux |
affected a10cede006f9614b465cf25609a8753efbfd45cc git |
Not specified |
| CNA |
Linux |
Linux |
affected efaf89a75a29b2d179bf4fe63ca62852e93ad620 git |
Not specified |
| CNA |
Linux |
Linux |
affected 5.10.247 5.10.265 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 5.15.197 5.15.216 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 6.1.159 6.1.183 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 6.6.117 6.6.152 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 6.12.58 6.12.104 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 5.4.302 5.5 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 6.17.8 6.18 semver |
Not specified |
| CNA |
Linux |
Linux |
affected 6.18 |
Not specified |
| CNA |
Linux |
Linux |
unaffected 6.18 semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 5.10.265 5.10.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 5.15.216 5.15.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 6.1.183 6.1.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 6.6.152 6.6.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 6.12.104 6.12.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 6.18.45 6.18.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 7.1.9 7.1.* semver |
Not specified |
| CNA |
Linux |
Linux |
unaffected 7.2 * original_commit_for_fix |
Not specified |
References
| Reference | Source | Link | Tags |
|---|
| git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8 |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3 |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576 |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091 |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700 |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c |
416baaa9-dc9f-4396-8d5f-8c081fb06d67 |
git.kernel.org |
|
| CVE Program record |
CVE.ORG |
www.cve.org |
canonical |
| NVD vulnerability detail |
NVD |
nvd.nist.gov |
canonical, analysis |
No vendor comments have been submitted for this CVE.
There are currently no legacy QID mappings associated with this CVE.