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Received: from viro by zeniv.linux.org.uk with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vJohq-00000009fzo-2VEk; Fri, 14 Nov 2025 07:58:54 +0000 Date: Fri, 14 Nov 2025 07:58:54 +0000 From: Al Viro To: Chris Mason Cc: Greg Kroah-Hartman , bot+bpf-ci@kernel.org, linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org, brauner@kernel.org, jack@suse.cz, raven@themaw.net, miklos@szeredi.hu, neil@brown.name, a.hindborg@kernel.org, linux-mm@kvack.org, linux-efi@vger.kernel.org, ocfs2-devel@lists.linux.dev, kees@kernel.org, rostedt@goodmis.org, linux-usb@vger.kernel.org, paul@paul-moore.com, casey@schaufler-ca.com, linuxppc-dev@lists.ozlabs.org, john.johansen@canonical.com, selinux@vger.kernel.org, borntraeger@linux.ibm.com, bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org, eddyz87@gmail.com, yonghong.song@linux.dev, ihor.solodrai@linux.dev Subject: Re: [functionfs] mainline UAF (was Re: [PATCH v3 36/50] functionfs: switch to simple_remove_by_name()) Message-ID: <20251114075854.GZ2441659@ZenIV> References: <20251111065520.2847791-37-viro@zeniv.linux.org.uk> <20754dba9be498daeda5fe856e7276c9c91c271999320ae32331adb25a47cd4f@mail.kernel.org> <20251111092244.GS2441659@ZenIV> <20251113092636.GX2441659@ZenIV> <2025111316-cornfield-sphinx-ba89@gregkh> <3984c9bd-2ac8-424e-9390-7170fdab3c03@meta.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3984c9bd-2ac8-424e-9390-7170fdab3c03@meta.com> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 77822C000C X-Stat-Signature: fjf7hkqgt1buph7ab8iy64du69t56rch X-Rspam-User: X-HE-Tag: 1763107141-973857 X-HE-Meta: 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 ExExedRA uqJOoHEcvY0FAPFQd/lZEG5yv4SL947mAcz/PjFWa4oLzklhfLiFJaBfzFwTWddwtuzQir2ph42rcgzk6FppLhYGg4PljNqoVPT8Pkt9NbONOzJroHiXbgSmHv1dQ5UyfNAtHE4h4NHzvXKhjT5d6UXWnozheaVSaDgXbp3IbG7gmuR9k6BsC3JY0Y/coNe2wUX0h71OSg0Dmt0rXqXr7Vx0W9MoxLVnErZHR6GrpdumD6zAEkwmADaXx24u+LYWYQHW+OGYw4K+e1j93MGAd4gDWyHLLTRMIzq6x3qnkgyJgaRVtI15CjYFGOXZgzamOuZBsm2eyY2AlEnURcirc87mnrhsDFbRI9lV2mjMDjerOWQPpwVot4rvFNyAFyd/cf2HP0TYp6Ya09EkxvYVPhZe60VdL7ehc7wddohQidtEAhd7Fq03XWpBQxQ5HAknRwmWnJ+N/ECsB1p+6mo4nfWVeirj7p0RSORgvubVsWMD4EWm4INumASbDwnf8lOb/L32/5+YXVaMrUItBK+PpnapzoRVB1bplbPf3MTCfD1dyIfb2ULaQJQAaeXD4BLblP1qB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Nov 13, 2025 at 09:16:52PM -0500, Chris Mason wrote: > ================================================================================ > BUG #2: Race condition in ffs_data_closed() > ================================================================================ > > In ffs_data_closed(), there's an unsynchronized state modification: > > static void ffs_data_closed(struct ffs_data *ffs) > { > ... > if (atomic_dec_and_test(&ffs->opened)) { > if (ffs->no_disconnect) { > ffs->state = FFS_DEACTIVATED; > ... > } else { > ffs->state = FFS_CLOSING; > ffs_data_reset(ffs); > } > } > if (atomic_read(&ffs->opened) < 0) { > ffs->state = FFS_CLOSING; > ffs_data_reset(ffs); > } > ... > } > > Can this race with concurrent state changes? The atomic_read() check is not > synchronized with the subsequent state assignment. Between the read and the > assignment, another thread could modify the state, potentially causing state > machine corruption or double cleanup via ffs_data_reset(). The atomic_read() check is utter bollocks. ->opened starts with 0, is bumped on opens on that fs and decremented when files are closed. How could it end up negative? Well might you ask - there's a call of ffs_data_closed() in ffs_kill_sb(). _There_ we are guaranteed that there's no opened files (we'd better, or there would be much worse problems), so ffs->opened is known to be 0 and ffs_data_closed() decrements it to -1. In other words, that couple of lines is executed on call from ffs_kill_sb() and only on that call. Which is to say, that thing is just an obfuscated if (s->s_fs_info) { struct ffs_data *ffs = s->s_fs_info; ffs->state = FFS_CLOSING; ffs_data_reset(ffs); ffs_data_put(ffs); } Not a race, just a highly unidiomatic code. While we are at it, after taking that mess out of ffs_data_closed() we no longer need to bump/drop ffs->ref in ffs_data_opened()/ffs_data_closed() - superblock owns a reference all along and it's *not* going away under an opened file.