bpf: Fix a sdiv overflow issue

Overview

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

bpf: Fix a sdiv overflow issue

Zac Ecob reported a problem where a bpf program may cause kernel crash due to the following error: Oops: divide error: 0000 [#1] PREEMPT SMP KASAN PTI

The failure is due to the below signed divide: LLONG_MIN/-1 where LLONG_MIN equals to -9,223,372,036,854,775,808. LLONG_MIN/-1 is supposed to give a positive number 9,223,372,036,854,775,808, but it is impossible since for 64-bit system, the maximum positive number is 9,223,372,036,854,775,807. On x86_64, LLONG_MIN/-1 will cause a kernel exception. On arm64, the result for LLONG_MIN/-1 is LLONG_MIN.

Further investigation found all the following sdiv/smod cases may trigger an exception when bpf program is running on x86_64 platform:

• LLONG_MIN/-1 for 64bit operation
• INT_MIN/-1 for 32bit operation
• LLONG_MIN%-1 for 64bit operation
• INT_MIN%-1 for 32bit operation where -1 can be an immediate or in a register.

On arm64, there are no exceptions:

• LLONG_MIN/-1 = LLONG_MIN
• INT_MIN/-1 = INT_MIN
• LLONG_MIN%-1 = 0
• INT_MIN%-1 = 0 where -1 can be an immediate or in a register.

Insn patching is needed to handle the above cases and the patched codes produced results aligned with above arm64 result. The below are pseudo codes to handle sdiv/smod exceptions including both divisor -1 and divisor 0 and the divisor is stored in a register.

sdiv: tmp = rX tmp += 1 /* [-1, 0] -> [0, 1] if tmp >(unsigned) 1 goto L2 if tmp == 0 goto L1 rY = 0 L1: rY = -rY; goto L3 L2: rY /= rX L3:

smod: tmp = rX tmp += 1 /* [-1, 0] -> [0, 1] if tmp >(unsigned) 1 goto L1 if tmp == 1 (is64 ? goto L2 : goto L3) rY = 0; goto L2 L1: rY %= rX L2: goto L4 // only when !is64 L3: wY = wY // only when !is64 L4:

[1] https://lore.kernel.org/bpf/tPJLTEh7S_DxFEqAI2Ji5MBSoZVg7_G-Py2iaZpAaWtM961fFTWtsnlzwvTbzBzaUzwQAoNATXKUlt0LZOFgnDcIyKCswAnAGdUF3LBrhGQ=@protonmail.com/

Affected

• Linux >= ec0e2da95f72d4a46050a4d994e4fe471474fd80 < 45f2bbe7bc08d716a07a40025c672cad93180903
• Linux >= ec0e2da95f72d4a46050a4d994e4fe471474fd80 < 4902a6a0dc593c82055fc8c9ada371bafe26c9cc
• Linux >= ec0e2da95f72d4a46050a4d994e4fe471474fd80 < d22e45a369afc7c28f11acfa5b5e8e478227ca5d
• Linux >= ec0e2da95f72d4a46050a4d994e4fe471474fd80 < 7dd34d7b7dcf9309fc6224caf4dd5b35bedddcb7
• Linux 6.6

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.