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| author | Xi Wang <xi.wang@gmail.com> | 2012-01-10 11:51:10 -0500 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@suse.de> | 2012-01-25 13:53:17 -0800 |
| commit | b4fb718e49d077ae66524193c749cf4b3b5f934e (patch) | |
| tree | f135a5e883f72e992c0101b2b0db61e5ead9f9e9 /fs | |
| parent | f2ab2a127d480ee20caf8620440ccf13c014447a (diff) | |
| download | lwn-b4fb718e49d077ae66524193c749cf4b3b5f934e.tar.gz lwn-b4fb718e49d077ae66524193c749cf4b3b5f934e.zip | |
ext4: fix undefined behavior in ext4_fill_flex_info()
commit d50f2ab6f050311dbf7b8f5501b25f0bf64a439b upstream.
Commit 503358ae01b70ce6909d19dd01287093f6b6271c ("ext4: avoid divide by
zero when trying to mount a corrupted file system") fixes CVE-2009-4307
by performing a sanity check on s_log_groups_per_flex, since it can be
set to a bogus value by an attacker.
sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
groups_per_flex = 1 << sbi->s_log_groups_per_flex;
if (groups_per_flex < 2) { ... }
This patch fixes two potential issues in the previous commit.
1) The sanity check might only work on architectures like PowerPC.
On x86, 5 bits are used for the shifting amount. That means, given a
large s_log_groups_per_flex value like 36, groups_per_flex = 1 << 36
is essentially 1 << 4 = 16, rather than 0. This will bypass the check,
leaving s_log_groups_per_flex and groups_per_flex inconsistent.
2) The sanity check relies on undefined behavior, i.e., oversized shift.
A standard-confirming C compiler could rewrite the check in unexpected
ways. Consider the following equivalent form, assuming groups_per_flex
is unsigned for simplicity.
groups_per_flex = 1 << sbi->s_log_groups_per_flex;
if (groups_per_flex == 0 || groups_per_flex == 1) {
We compile the code snippet using Clang 3.0 and GCC 4.6. Clang will
completely optimize away the check groups_per_flex == 0, leaving the
patched code as vulnerable as the original. GCC keeps the check, but
there is no guarantee that future versions will do the same.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/ext4/super.c | 7 |
1 files changed, 3 insertions, 4 deletions
diff --git a/fs/ext4/super.c b/fs/ext4/super.c index f27e045df7a6..f1e7077a06a3 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -1705,17 +1705,16 @@ static int ext4_fill_flex_info(struct super_block *sb) struct ext4_group_desc *gdp = NULL; ext4_group_t flex_group_count; ext4_group_t flex_group; - int groups_per_flex = 0; + unsigned int groups_per_flex = 0; size_t size; int i; sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex; - groups_per_flex = 1 << sbi->s_log_groups_per_flex; - - if (groups_per_flex < 2) { + if (sbi->s_log_groups_per_flex < 1 || sbi->s_log_groups_per_flex > 31) { sbi->s_log_groups_per_flex = 0; return 1; } + groups_per_flex = 1 << sbi->s_log_groups_per_flex; /* We allocate both existing and potentially added groups */ flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) + |