Subject: CVE-2024-35784: btrfs: fix deadlock with fiemap and extent locking

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix deadlock with fiemap and extent locking

While working on the patchset to remove extent locking I got a lockdep
splat with fiemap and pagefaulting with my new extent lock replacement
lock.

This deadlock exists with our normal code, we just don't have lockdep
annotations with the extent locking so we've never noticed it.

Since we're copying the fiemap extent to user space on every iteration
we have the chance of pagefaulting.  Because we hold the extent lock for
the entire range we could mkwrite into a range in the file that we have
mmap'ed.  This would deadlock with the following stack trace

[<0>] lock_extent+0x28d/0x2f0
[<0>] btrfs_page_mkwrite+0x273/0x8a0
[<0>] do_page_mkwrite+0x50/0xb0
[<0>] do_fault+0xc1/0x7b0
[<0>] __handle_mm_fault+0x2fa/0x460
[<0>] handle_mm_fault+0xa4/0x330
[<0>] do_user_addr_fault+0x1f4/0x800
[<0>] exc_page_fault+0x7c/0x1e0
[<0>] asm_exc_page_fault+0x26/0x30
[<0>] rep_movs_alternative+0x33/0x70
[<0>] _copy_to_user+0x49/0x70
[<0>] fiemap_fill_next_extent+0xc8/0x120
[<0>] emit_fiemap_extent+0x4d/0xa0
[<0>] extent_fiemap+0x7f8/0xad0
[<0>] btrfs_fiemap+0x49/0x80
[<0>] __x64_sys_ioctl+0x3e1/0xb50
[<0>] do_syscall_64+0x94/0x1a0
[<0>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

I wrote an fstest to reproduce this deadlock without my replacement lock
and verified that the deadlock exists with our existing locking.

To fix this simply don't take the extent lock for the entire duration of
the fiemap.  This is safe in general because we keep track of where we
are when we're searching the tree, so if an ordered extent updates in
the middle of our fiemap call we'll still emit the correct extents
because we know what offset we were on before.

The only place we maintain the lock is searching delalloc.  Since the
delalloc stuff can change during writeback we want to lock the extent
range so we have a consistent view of delalloc at the time we're
checking to see if we need to set the delalloc flag.

With this patch applied we no longer deadlock with my testcase.

The Linux kernel CVE team has assigned CVE-2024-35784 to this issue.

Affected and fixed versions
===========================

	Fixed in 6.6.24 with commit ded566b4637f
	Fixed in 6.7.12 with commit 89bca7fe6382
	Fixed in 6.8 with commit b0ad381fa769

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35784
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/btrfs/extent_io.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/ded566b4637f1b6b4c9ba74e7d0b8493e93f19cf
	https://git.kernel.org/stable/c/89bca7fe6382d61e88c67a0b0e7bce315986fb8b
	https://git.kernel.org/stable/c/b0ad381fa7690244802aed119b478b4bdafc31dd