smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

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Summary

CVECVE-2026-89638
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-11 20:19:49 UTC
Updated2026-09-11 20:19:49 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions When a file has the setuid or setgid bit set and is written to, the VFS strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID together with an ATTR_MODE carrying the already-cleared mode. Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped ATTR_MODE in that case: /* skip mode change if it's just for clearing setuid/setgid */ if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID)) attrs->ia_valid &= ~ATTR_MODE; This is fine for the default mount, where the mode is only emulated via the DOS read-only attribute and cannot represent the setuid/setgid bits anyway. However, with the "cifsacl" or "modefromsid" mount options the mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with the SMB3.1.1 POSIX extensions the mode is sent to the server directly, and with the SMB1 Unix extensions (cifs_setattr_unix) the mode is sent via CIFSSMBUnixSetPathInfo(). In all those cases dropping ATTR_MODE means the cleared mode is never pushed to the server, so the setuid/setgid bit survives the write. This is a security issue: on local filesystems the setuid bit is stripped when a file is written, but over these cifs.ko mounts the bit persists on the server, potentially allowing an unexpected privilege escalation on subsequent execution. Fix this in two places: 1. cifs_setattr_nounix(): only take the "skip mode change" shortcut when the mode is emulated via the DOS read-only attribute (i.e. neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are in effect), so that the cleared mode is propagated to the server in the ACL / POSIX cases. 2. cifs_setattr_unix(): this function is only called when Unix extensions are in effect, so the mode is always stored on the server. Remove the shortcut entirely so that the cleared mode is always pushed.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected d32c4f2626acc385d5187bd5c0c24f442328dc60 b10015807e4c628095d1d1d1c9307efc8cdd9e1b git Not specified
CNA Linux Linux affected d32c4f2626acc385d5187bd5c0c24f442328dc60 69bfe810ecd1e0387d8975d711cd326341d13c6e git Not specified
CNA Linux Linux affected d32c4f2626acc385d5187bd5c0c24f442328dc60 b8e5dc4f95e5484159b343903f302eb6d783f2e6 git Not specified
CNA Linux Linux affected 2.6.24 Not specified
CNA Linux Linux unaffected 2.6.24 semver Not specified
CNA Linux Linux unaffected 6.18.51 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.4 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/b10015807e4c628095d1d1d1c9307efc8cdd9e1b 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/b8e5dc4f95e5484159b343903f302eb6d783f2e6 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/69bfe810ecd1e0387d8975d711cd326341d13c6e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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