From: Jan Kara <jack@suse.cz>
To: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Muchun Song <muchun.song@linux.dev>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Christian Brauner <brauner@kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Hugh Dickins <hughd@google.com>,
Andy Lutomirski <luto@kernel.org>, Jan Kara <jack@suse.cz>,
linux-fsdevel@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH v3 3/3] mm: enforce the mapping_map_writable() check after call_mmap()
Date: Wed, 11 Oct 2023 11:46:27 +0200 [thread overview]
Message-ID: <20231011094627.3xohlpe4gm2idszm@quack3> (raw)
In-Reply-To: <d2748bc4077b53c60bcb06fccaf976cb2afee345.1696709413.git.lstoakes@gmail.com>
On Sat 07-10-23 21:51:01, Lorenzo Stoakes wrote:
> In order for an F_SEAL_WRITE sealed memfd mapping to have an opportunity to
> clear VM_MAYWRITE in seal_check_write() we must be able to invoke either
> the shmem_mmap() or hugetlbfs_file_mmap() f_ops->mmap() handler to do so.
>
> We would otherwise fail the mapping_map_writable() check before we had
> the opportunity to clear VM_MAYWRITE.
>
> However, the existing logic in mmap_region() performs this check BEFORE
> calling call_mmap() (which invokes file->f_ops->mmap()). We must enforce
> this check AFTER the function call.
>
> In order to avoid any risk of breaking call_mmap() handlers which assume
> this will have been done first, we continue to mark the file writable
> first, simply deferring enforcement of it failing until afterwards.
>
> This enables mmap(..., PROT_READ, MAP_SHARED, fd, 0) mappings for memfd's
> sealed via F_SEAL_WRITE to succeed, whereas previously they were not
> permitted.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
...
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6f6856b3267a..9fbee92aaaee 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2767,17 +2767,25 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> vma->vm_pgoff = pgoff;
>
> if (file) {
> - if (is_shared_maywrite(vm_flags)) {
> - error = mapping_map_writable(file->f_mapping);
> - if (error)
> - goto free_vma;
> - }
> + int writable_error = 0;
> +
> + if (vma_is_shared_maywrite(vma))
> + writable_error = mapping_map_writable(file->f_mapping);
>
> vma->vm_file = get_file(file);
> error = call_mmap(file, vma);
> if (error)
> goto unmap_and_free_vma;
>
> + /*
> + * call_mmap() may have changed VMA flags, so retry this check
> + * if it failed before.
> + */
> + if (writable_error && vma_is_shared_maywrite(vma)) {
> + error = writable_error;
> + goto close_and_free_vma;
> + }
Hum, this doesn't quite give me a peace of mind ;). One bug I can see is
that if call_mmap() drops the VM_MAYWRITE flag, we seem to forget to drop
i_mmap_writeable counter here?
I've checked why your v2 version broke i915 and I think the reason maybe
has nothing to do with i915. Just in case call_mmap() failed, it ended up
jumping to unmap_and_free_vma which calls mapping_unmap_writable() but we
didn't call mapping_map_writable() yet so the counter became imbalanced.
So I'd be for returning to v2 version, just fix up the error handling
paths...
Honza
> +
> /*
> * Expansion is handled above, merging is handled below.
> * Drivers should not alter the address of the VMA.
> --
> 2.42.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2023-10-11 9:46 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-07 20:50 [PATCH v3 0/3] permit write-sealed memfd read-only shared mappings Lorenzo Stoakes
2023-10-07 20:50 ` [PATCH v3 1/3] mm: drop the assumption that VM_SHARED always implies writable Lorenzo Stoakes
2023-10-07 20:51 ` [PATCH v3 2/3] mm: update memfd seal write check to include F_SEAL_WRITE Lorenzo Stoakes
2023-10-07 20:51 ` [PATCH v3 3/3] mm: enforce the mapping_map_writable() check after call_mmap() Lorenzo Stoakes
2023-10-11 9:46 ` Jan Kara [this message]
2023-10-11 18:14 ` Lorenzo Stoakes
2023-10-12 8:38 ` Jan Kara
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