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Fixed out-of-bounds read of extended attribute values in filestat() - #6429
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Dhanu-dynamic wants to merge 1 commit into
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Dhanu-dynamic wants to merge 1 commit into
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The "xattr" branch of filestat() reads each extended attribute value
into an uninitialized stack buffer and then uses it as a C string:
char data[CF_BUFSIZE];
int datasize = lgetxattr(path, current, data, sizeof(data));
...
if (!StringIsPrintable(data))
...
BufferPrintf(printattr, "%s=%s", current, data);
lgetxattr() returns raw bytes and never adds a terminating NUL. An
unprivileged user can set user.* attributes on files they own, so when
a policy run as root calls filestat() on such a file, a printable value
of exactly 4096 bytes makes StringIsPrintable() read past the end of
data. Shorter values are followed by whatever stale stack bytes come
before the next NUL, which either makes a valid attribute get skipped
as non-printable or copies stack contents into the returned string.
Read at most sizeof(data) - 1 bytes and terminate the value at
datasize. CopyFileExtendedAttributesDisk() in files_copy.c, the other
lgetxattr() caller, handles values by length and is not affected.
Changelog: Title
Signed-off-by: Dhanraj E <dhanraj@labs.digiscrypt.com>
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Thank you for submitting a PR! Maybe @craigcomstock can review this? |
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Repro: as an unprivileged user,
setfattr -n user.x -v "$(head -c 4096 /dev/zero | tr '\0' A)" ~/f, then have a root policy evaluatefilestat("/home/user/f", "xattr"). Under ASan this is a stack-buffer-overflow read inStringIsPrintable(). A short value such asuser.x=abcis followed by stale stack bytes, so it is either dropped as non-printable or returned with stack contents appended.Cause:
lgetxattr()fillsdata[CF_BUFSIZE]with raw bytes and never NUL-terminates, butFnCallFileStatDetails()then usesdataas a C string.Fix: read at most
sizeof(data) - 1bytes and terminate atdatasize.CopyFileExtendedAttributesDisk()infiles_copy.chandles values by length and is not affected.I checked this by running the same loop against real extended attributes under ASan: the over-read before the change,
user.x=abcreturned correctly after it. The unit test suite still passes. I didn't add a unit test because it would depend on the CI filesystem supportinguser.*xattrs.