From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793112106; b=vJWuBoq4HtPuLCNZcsUIsDkIwoPsRT4/FRCB1iquB1W5iAvLJE2UZHl6LH8cbAufzfVk inwasKi2Yizy4a23/KDdGuZOZjOi/H3bmtLl89fniNJd0AIow9qEgZtPOnTYuccVKTGv7 5CQP4NwmQFu2WG7ZJkOMeThig66hDrQ6B7X6Az5rySDuCXYO97DJKwiNMWk4TDS0p/LeW 7A8bDkRIvpQEdIm9buSNRvwMVTipYHgqtu9u3f/VL6XO3OI3rGhgidrf6fU7DND1PiMhJ RYKqHk5p+kYvFkleol5bLl6Q7DtMDqnhQLW+B4MikOYv2OTyLDDLNVD1pRTJPVoRk9Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793112106; h=Received:DKIM-Signature:From:To:Cc:Subject:In-Reply-To:References: Date:Message-ID:User-Agent:MIME-Version:Content-Type; bh=Cxbcd97TigLbOc2nsbdYTtwGKr7cyQITuDlutqpOwp0=; b=aQWoFWncYIly05m2wm8N8TcnU4qli123A04TBWdwMxkibL+tPmzuwdvgz8KkjeYCP2Fh a0/UE05pck6F7gK5k9juD8LUvV7SDMxMDLxx9+K7bYgcDZBXVeL1GEaKKpJRCKKM0A/Je eGRNpE4W2WancAtRCKLZN3YYlKXVMIwKg697vFBRVazVZNBww9QkKrTawNe8GKGNADMDw yUSyfUllzKuX88ljONv1M52IkmfYaJEXDos1JYaSQb6zSD0zU/BuQ+G7l9T0X5M5pdqGv GAMAmTLIj/vlGKVhfQNon/iHn8aPOS2cX613oE+0vcFks8HAxFkqVDEA4iNsfn6e7AA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="F/aE3Kg9"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 54B82606A31E; Tue, 27 Oct 2026 14:41:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=F/aE3Kg9 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 61A8B1F01825; Tue, 27 Oct 2026 14:41:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793112105; bh=Cxbcd97TigLbOc2nsbdYTtwGKr7cyQITuDlutqpOwp0=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=F/aE3Kg94sj9fXVFeMOGEhnFPh7IPM73CFA/LmgAAZfItP5Nw7PY1j3LmhUrie3RF WWOkZE/CrqgRjQtrY5czn2Xyhnu7Jhr5cpxZIRN6ucwFL7953eyR9cc+xJIfYeqrtx x427FQ040+6sFoWm3ZQYEETyJqIrMT+OQrZXuAnXJaqqyPl1iBQjI9L2UIFGYkQFmj rXT+tzW5xd2XQNYXVDL7CnDknr0SBZMXfP2tLX56GVF5sYE25tWUnjur3LSUQsNAM/ e9FZ57Wpnn2LjlHGFOu9V+hCx07zuXfQscJu7g4AtCiXI1sCKypZwCMKvsYpJoK5pk vtVZ2ljYGxZvA== From: Pratyush Yadav To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, stable@vger.kernel.org, Chris Mason Subject: Re: [PATCH 03/46] mm/memfd: check for real shmem inodes before accessing shmem state In-Reply-To: <20261027-fixes_core-v1-3-6f74bec9a5bb@kernel.org> (David Hildenbrand's message of "Tue, 27 Oct 2026 00:17:23 +0100") References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-3-6f74bec9a5bb@kernel.org> Date: Tue, 27 Oct 2026 15:41:34 +0100 Message-ID: <2vxzh5i7p5nl.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain Hi David, On Tue, Oct 27 2026, David Hildenbrand (Arm) wrote: > shmem_file() only checks a file's backing mapping. That mapping can belong > to another file, as with a Coda file backed by a tmpfs container after > mmap(). It does not establish that file_inode(file) is a shmem inode. > > Seal handling and memfd preservation use this check before accessing > shmem inode state, so they can access an inode of the wrong type. > > Add file_inode_is_shmem() to check the inode's file operations in both > paths. This identifies regular shmem inodes, including those used by > memfds and ordinary tmpfs files, independently of wrapper file operations. Why not update shmem_file() to do this check instead? Having a shmem_file() that might not always return the right result on a file seems like a landmine. For example, you missed check_memfd_seals() in udmabuf or memfd_pin_folios() in gup. Plus, someone later down the line might use shmem_file() without knowing this subtle problem, and we probably would miss it in review. For callers that only care about the mapping should then use shmem_mapping(). Perhaps that would be collapse_file() but TBH I don't really understand khugepaged all that much. All other callers of shmem_file() seem to care about the actual inode. > > Fixes: c49fcfcda8b5 ("mm/shmem: update file sealing comments and file checking") > Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd") > Cc: stable@vger.kernel.org > Reported-by: Chris Mason > Signed-off-by: David Hildenbrand (Arm) > --- > include/linux/shmem_fs.h | 6 ++++++ > mm/memfd.c | 8 +++++--- > mm/memfd_luo.c | 2 +- > mm/shmem.c | 5 +++++ > 4 files changed, 17 insertions(+), 4 deletions(-) > > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h > index a7c7a96a7cbf9..2ceca7993e4f5 100644 > --- a/include/linux/shmem_fs.h > +++ b/include/linux/shmem_fs.h > @@ -117,8 +117,14 @@ extern unsigned long shmem_get_unmapped_area(struct file *, unsigned long addr, > unsigned long len, unsigned long pgoff, unsigned long flags); > extern int shmem_lock(struct file *file, int lock, struct ucounts *ucounts); > #ifdef CONFIG_SHMEM > +bool file_inode_is_shmem(const struct inode *inode); > bool shmem_mapping(const struct address_space *mapping); > #else > +static inline bool file_inode_is_shmem(const struct inode *inode) > +{ > + return false; > +} > + > static inline bool shmem_mapping(const struct address_space *mapping) > { > return false; > diff --git a/mm/memfd.c b/mm/memfd.c > index 0f6fff004f5e4..7d2da23157003 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -211,12 +211,14 @@ static int memfd_wait_for_pins(struct address_space *mapping) > > static unsigned int *memfd_file_seals_ptr(struct file *file) > { > - if (shmem_file(file)) > - return &SHMEM_I(file_inode(file))->seals; > + struct inode *inode = file_inode(file); > + > + if (file_inode_is_shmem(inode)) > + return &SHMEM_I(inode)->seals; > > #ifdef CONFIG_HUGETLBFS > if (is_file_hugepages(file)) > - return &HUGETLBFS_I(file_inode(file))->seals; > + return &HUGETLBFS_I(inode)->seals; > #endif > > return NULL; > diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c > index 59de210bee5f9..c0811407ce397 100644 > --- a/mm/memfd_luo.c > +++ b/mm/memfd_luo.c > @@ -582,7 +582,7 @@ static bool memfd_luo_can_preserve(struct liveupdate_file_handler *handler, > { > struct inode *inode = file_inode(file); > > - return shmem_file(file) && !inode->i_nlink; > + return file_inode_is_shmem(inode) && !inode->i_nlink; > } > > static unsigned long memfd_luo_get_id(struct file *file) > diff --git a/mm/shmem.c b/mm/shmem.c > index fadf23f9e21ed..0d41aac125df9 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -275,6 +275,11 @@ static const struct vm_operations_struct shmem_vm_ops; > static const struct vm_operations_struct shmem_anon_vm_ops; > static struct file_system_type shmem_fs_type; > > +bool file_inode_is_shmem(const struct inode *inode) > +{ > + return inode->i_fop == &shmem_file_operations; > +} > + > bool shmem_mapping(const struct address_space *mapping) > { > return mapping->a_ops == &shmem_aops; -- Regards, Pratyush Yadav