Overview
About vulnerability
In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject negative const offsets for buffer pointers
The verifier rejects variable offsets for PTR_TO_TP_BUFFER and PTR_TO_BUF accesses, but it currently accepts a constant negative offset produced by pointer arithmetic.
Commit 022ac0750883 (“bpf: use reg->var_off instead of reg->off for pointers”) moved constant pointer offsets from reg->off to reg->var_off. However, __check_buffer_access() continued to check only the instruction offset. An access with reg->var_off equal to -8 and an instruction offset of zero therefore passes verification.
For writable raw tracepoints, the access end is also calculated from the unsigned reg->var_off.value. An eight-byte access starting at -8 wraps the calculated end to zero, allowing the program to load and attach without increasing max_tp_access.
After ensuring that reg->var_off is constant, calculate the effective access start using signed arithmetic and reject it when it is negative. Use the validated start to calculate the access end for both PTR_TO_TP_BUFFER and PTR_TO_BUF.
Details
- Affected product:
- Debian 10 ELS
- Affected packages:
- linux @ 4.19.0