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

[Pedro Falcato <pfalcato@suse.de>]

On Tue, Jul 28, 2026 at 09:52:26AM +0200, Christian Brauner 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@vger.kernel.org
> > Signed-off-by: Pedro Falcato <pfalcato@suse.de>
> 
> Did that go in?

Yes, this is e187bc02f8fa4226d62814592cf064ee4557c470 in v7.2-rc3.

-- 
Pedro