CVE-2026-64294

In the Linux kernel, the following vulnerability has been resolved:

mm: do file ownership checks with the proper mount idmap

Ever since idmapped mounts were introduced, inode ownership checks (for

side-channel protection) in mincore() and madvise(MADV_PAGEOUT) were done

against the nop_mnt_idmap, which completely ignores the file's mount's

idmap. This results in odd edgecases like:

1) mount/bind-mount with an idmap userA:userB:1

2) userB runs an owner_or_capable() check on file that is owned by userA

on-disk/in-memory, but owned by userB after idmap translation

3) owner_or_capable() mysteriously fails as the correct idmap wasn't supplied

In the case of mincore/madvise MADV_PAGEOUT, this is usually benign,

because file_permission(file, MAY_WRITE) will probably succeed, as it uses

the proper idmap internally, but it does not need to be the case on e.g a

0444 file where even the owner itself doesn't have permissions to write to

it.

Since this is clearly not trivial to get right, introduce a

file_owner_or_capable() that can carry the correct semantics, and switch

the various users in mm to it.

The issue was found by manual code inspection & an off-list discussion

with Jan Kara.