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

[Christian Brauner]

On 2026-06-26 16:19:18+02:00, Jan Kara wrote:
> On Thu 25-06-26 16:38:53, 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>
> 
> This looks good to me. I'm a bit curious why Christian initially (in 2021)
> used init_user_ns here instead of the file namespace... Anyway feel free to
> add:

Back when this was added only the do_mincore() codepath existed and that
was intentionally left unconverted because it exposes the cache
residency status. So it was effectively a massive side-channel.

Both fd3b1bc3c86e ("mm/madvise: fix madvise_pageout for private file mappings")
and specifically cachestat() came way after all that.

I'm otherwise fine with the change.

Reviewed-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>