Bug 2502461 (CVE-2026-63909) - CVE-2026-63909 kernel: ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops
Summary: CVE-2026-63909 kernel: ksmbd: OOB read regression in smb_check_perm_dacl() AC...
Keywords:
Status: NEW
Alias: CVE-2026-63909
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
unspecified
unspecified
Target Milestone: ---
Assignee: Product Security DevOps Team
QA Contact:
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Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2026-07-19 16:09 UTC by OSIDB Bzimport
Modified: 2026-07-21 11:30 UTC (History)
2 users (show)

Fixed In Version:
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Description OSIDB Bzimport 2026-07-19 16:09:25 UTC
In the Linux kernel, the following vulnerability has been resolved:

ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

Commit d07b26f39246 ("ksmbd: require minimum ACE size in
smb_check_perm_dacl()") introduced a transposed bounds check:

    if (offsetof(struct smb_ace, sid) + aces_size < CIFS_SID_BASE_SIZE)

Since offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates
to `aces_size < 0`. Because `aces_size` is always non-negative, this
check becomes dead code and never breaks the loop.

Worse, that commit removed the old 4-byte guard, meaning the loop now
reads `ace->size` (offset 2) even when `aces_size` is 0-3 bytes. This
re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation
during subsequent SMB2_CREATE operations.

Fix this by properly transposing the comparison to require at least
16 bytes (8-byte offset + 8-byte SID base), matching the correct form
used in smb_inherit_dacl().

Comment 1 Mauro Matteo Cascella 2026-07-21 11:26:41 UTC
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026071945-CVE-2026-63909-d312@gregkh/T


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