Subject: CVE-2024-47741: btrfs: fix race setting file private on concurrent lseek using same fd

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

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

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

btrfs: fix race setting file private on concurrent lseek using same fd

When doing concurrent lseek(2) system calls against the same file
descriptor, using multiple threads belonging to the same process, we have
a short time window where a race happens and can result in a memory leak.

The race happens like this:

1) A program opens a file descriptor for a file and then spawns two
   threads (with the pthreads library for example), lets call them
   task A and task B;

2) Task A calls lseek with SEEK_DATA or SEEK_HOLE and ends up at
   file.c:find_desired_extent() while holding a read lock on the inode;

3) At the start of find_desired_extent(), it extracts the file's
   private_data pointer into a local variable named 'private', which has
   a value of NULL;

4) Task B also calls lseek with SEEK_DATA or SEEK_HOLE, locks the inode
   in shared mode and enters file.c:find_desired_extent(), where it also
   extracts file->private_data into its local variable 'private', which
   has a NULL value;

5) Because it saw a NULL file private, task A allocates a private
   structure and assigns to the file structure;

6) Task B also saw a NULL file private so it also allocates its own file
   private and then assigns it to the same file structure, since both
   tasks are using the same file descriptor.

   At this point we leak the private structure allocated by task A.

Besides the memory leak, there's also the detail that both tasks end up
using the same cached state record in the private structure (struct
btrfs_file_private::llseek_cached_state), which can result in a
use-after-free problem since one task can free it while the other is
still using it (only one task took a reference count on it). Also, sharing
the cached state is not a good idea since it could result in incorrect
results in the future - right now it should not be a problem because it
end ups being used only in extent-io-tree.c:count_range_bits() where we do
range validation before using the cached state.

Fix this by protecting the private assignment and check of a file while
holding the inode's spinlock and keep track of the task that allocated
the private, so that it's used only by that task in order to prevent
user-after-free issues with the cached state record as well as potentially
using it incorrectly in the future.

The Linux kernel CVE team has assigned CVE-2024-47741 to this issue.

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

	Issue introduced in 6.2 with commit 3c32c7212f16 and fixed in 6.6.54 with commit f56a6d9c267e
	Issue introduced in 6.2 with commit 3c32c7212f16 and fixed in 6.10.13 with commit a412ca489ac2
	Issue introduced in 6.2 with commit 3c32c7212f16 and fixed in 6.11.2 with commit 33d1310d4496
	Issue introduced in 6.2 with commit 3c32c7212f16 and fixed in 6.12-rc1 with commit 7ee85f5515e8

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-2024-47741
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/btrfs_inode.h
	fs/btrfs/ctree.h
	fs/btrfs/file.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/f56a6d9c267ec7fa558ede7755551c047b1034cd
	https://git.kernel.org/stable/c/a412ca489ac27b9d0e603499315b7139c948130d
	https://git.kernel.org/stable/c/33d1310d4496e904123dab9c28b2d8d2c1800f97
	https://git.kernel.org/stable/c/7ee85f5515e86a4e2a2f51969795920733912bad