Re: [PATCH] mm: do file ownership checks with the proper mount idmap

[David Hildenbrand (Arm)]

On 6/25/26 17:38, Pedro Falcato wrote:
> 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.
> 
> Fixes: 9caccd41541a ("fs: introduce MOUNT_ATTR_IDMAP")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Pedro Falcato <pfalcato@xxxxxxx>

MM side LGTM

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David