CVE-2022-50478

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

nilfs2: fix shift-out-of-bounds/overflow in nilfs_sb2_bad_offset()

Patch series nilfs2: fix UBSAN shift-out-of-bounds warnings on mount time.

The first patch fixes a bug reported by syzbot, and the second one fixes the remaining bug of the same kind. Although they are triggered by the same super block data anomaly, I divided it into the above two because the details of the issues and how to fix it are different.

Both are required to eliminate the shift-out-of-bounds issues at mount time.

This patch (of 2):

If the block size exponent information written in an on-disk superblock is corrupted, nilfs_sb2_bad_offset helper function can trigger shift-out-of-bounds warning followed by a kernel panic (if panic_on_warn is set):

shift exponent 38983 is too large for 64-bit type unsigned long long Call Trace: __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0x1b1/0x28e lib/dump_stack.c:106 ubsan_epilogue lib/ubsan.c:151 [inline] __ubsan_handle_shift_out_of_bounds+0x33d/0x3b0 lib/ubsan.c:322 nilfs_sb2_bad_offset fs/nilfs2/the_nilfs.c:449 [inline] nilfs_load_super_block+0xdf5/0xe00 fs/nilfs2/the_nilfs.c:523 init_nilfs+0xb7/0x7d0 fs/nilfs2/the_nilfs.c:577 nilfs_fill_super+0xb1/0x5d0 fs/nilfs2/super.c:1047 nilfs_mount+0x613/0x9b0 fs/nilfs2/super.c:1317 …

In addition, since nilfs_sb2_bad_offset() performs multiplication without considering the upper bound, the computation may overflow if the disk layout parameters are not normal.

This fixes these issues by inserting preliminary sanity checks for those parameters and by converting the comparison from one involving multiplication and left bit-shifting to one using division and right bit-shifting.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/1012ff77284e3bec0ec0a35a820b03ec43dec2cc
• https://git.kernel.org/stable/c/610a2a3d7d8be3537458a378ec69396a76c385b6
• https://git.kernel.org/stable/c/62d11ec205ef14d8acf172cfc9904fdbf200025a
• https://git.kernel.org/stable/c/6b0ea3df56cccd53398d0289f399f19d43136b2e
• https://git.kernel.org/stable/c/9b3ba54025357440d6c4414c670984f628c6f6bf
• https://git.kernel.org/stable/c/a6f89b10042baca218c8598d6db5a44c7e32625f
• https://git.kernel.org/stable/c/b47f5c579c8186f7f5ab5e4254e0734ea5b7bf7a
• https://git.kernel.org/stable/c/d464b035c0613856d012cf1704879d3ff3f057fb
• https://git.kernel.org/stable/c/d706485dffbbbf848e681edda29c7a46ac55698c