CVE-2024-35798 btrfs: fix race in read_extent_buffer_pages()

2024-05-1713:23:08

Linux

www.cve.org

linux kernel

btrfs

vulnerability

fixed

race

read extent buffer

memory overwrite

tree-checker

concurrent reads

uptodate

reading bit fix

7.5 High

AI Score

Confidence

High

0.0004 Low

EPSS

Percentile

15.5%

JSON

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

btrfs: fix race in read_extent_buffer_pages()

There are reports from tree-checker that detects corrupted nodes,
without any obvious pattern so possibly an overwrite in memory.
After some debugging it turns out there’s a race when reading an extent
buffer the uptodate status can be missed.

To prevent concurrent reads for the same extent buffer,
read_extent_buffer_pages() performs these checks:

/* (1) */
if (test_bit(EXTENT_BUFFER_UPTODATE, &eb-&gt;bflags))
    return 0;

/* (2) */
if (test_and_set_bit(EXTENT_BUFFER_READING, &eb-&gt;bflags))
    goto done;

At this point, it seems safe to start the actual read operation. Once
that completes, end_bbio_meta_read() does

/* (3) */
set_extent_buffer_uptodate(eb);

/* (4) */
clear_bit(EXTENT_BUFFER_READING, &eb-&gt;bflags);

Normally, this is enough to ensure only one read happens, and all other
callers wait for it to finish before returning. Unfortunately, there is
a racey interleaving:

Thread A | Thread B | Thread C
---------+----------+---------
   (1)   |          |
         |    (1)   |
   (2)   |          |
   (3)   |          |
   (4)   |          |
         |    (2)   |
         |          |    (1)

When this happens, thread B kicks of an unnecessary read. Worse, thread
C will see UPTODATE set and return immediately, while the read from
thread B is still in progress. This race could result in tree-checker
errors like this as the extent buffer is concurrently modified:

BTRFS critical (device dm-0): corrupted node, root=256
block=8550954455682405139 owner mismatch, have 11858205567642294356
expect [256, 18446744073709551360]

Fix it by testing UPTODATE again after setting the READING bit, and if
it’s been set, skip the unnecessary read.

[ minor update of changelog ]

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/btrfs/extent_io.c"
    ],
    "versions": [
      {
        "version": "d7172f52e993",
        "lessThan": "0427c8ef8bbb",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "d7172f52e993",
        "lessThan": "3a25878a3378",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "d7172f52e993",
        "lessThan": "2885d54af2c2",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "d7172f52e993",
        "lessThan": "ef1e68236b91",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/btrfs/extent_io.c"
    ],
    "versions": [
      {
        "version": "6.5",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "6.5",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.6.24",
        "lessThanOrEqual": "6.6.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.7.12",
        "lessThanOrEqual": "6.7.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.8.3",
        "lessThanOrEqual": "6.8.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.9",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]