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"jk@ozlabs.org" , linux-fsdevel@vger.kernel.org Date: Sun, 16 Mar 2025 10:26:12 -0400 In-Reply-To: <20250316-vergibt-hausrat-b23d525a1d24@brauner> References: <465d1d23-3b36-490e-b0dd-74889d17fa4c@tnxip.de> <20250311-visite-rastplatz-d1fdb223dc10@brauner> <814a257530ad5e8107ce5f48318ab43a3ef1f783.camel@HansenPartnership.com> <7bdcc2c5d8022d2f1a7ec23c0351f7816d4464c8.camel@HansenPartnership.com> <20250315-allemal-fahrbahn-9afc7bc0008d@brauner> <20250316-vergibt-hausrat-b23d525a1d24@brauner> Autocrypt: addr=James.Bottomley@HansenPartnership.com; prefer-encrypt=mutual; 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 8mT2Eh9SkH+YVOmaKsajgZgrBxA7fWmGoxXswEVxJIFj3vW7yNc0C5HaUdYa5iGOMs4kg2ht4s7yy7NRQuh7BifWjo6BQ6k4S1H+6axZucxhSV1L6zN9d+lr3Xo/vy1unzA== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.3 Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sun, 2025-03-16 at 07:46 +0100, Christian Brauner wrote: > On Sat, Mar 15, 2025 at 02:41:43PM -0400, James Bottomley wrote: [...] > > However, there's another problem: the mntput after kernel_file_open > > may synchronously call cleanup_mnt() (and thus deactivate_super()) > > if the open fails because it's marked MNT_INTERNAL, which is caused > > by SB_KERNMOUNT.=C2=A0 I fixed this just by not passing the SB_KERNMOUN= T > > flag, which feels a bit hacky. >=20 > It actually isn't. We know that vfs_kern_mount() will always > resurface the single superblock that's exposed to userspace because > we've just taken a reference to it earlier in efivarfs_pm_notify(). > So that SB_KERNMOUNT flag is ignored because no new superblock is > allocated. It would only matter if we'd end up allocating a new > superblock which we never do. I agree with the above: fc->sb_flags never propagates to the existing superblock. However, nothing propagates the superblock flags back to fc->sb_flags either. The check in vfs_create_mount() is on fc- >sb_flags. Since the code is a bit hard to follow I added a printk on the path.mnt flags and sure enough it comes back with MNT_INTERNAL when SB_KERNMOUNT is set. > And if we did it would be a bug because the superblock we allocate > could be reused at any time if a userspace task mounts efivarfs > before efivarfs_pm_notify() has destroyed it (or the respective > workqueue). But that superblock would then have SB_KERNMOUNT for > something that's not supposed to be one. True, but the flags don't propagate to the superblock, so no bug. > And whether or not that helper mount has MNT_INTERNAL is immaterial > to what you're doing here afaict. I think the problem is the call chain mntput() -> mntput_no_expire() which directly calls cleanup_mnt() -> deactivate_super() if that flag is set. Though I don't see that kernel_file_open() could ever fail except for some catastrophic reason like out of memory, so perhaps it isn't worth quibbling about. > So not passing the SB_KERNMOUNT flag is the right thing (see devtmpfs > as well). You could slap a comment in here explaining that we never > allocate a new superblock so it's clear to people not familiar with > this particular code. OK, so you agree that the code as written looks correct? Even if we don't necessarily quite agree on why. Regards, James