Description

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

s390/bpf: Fix bpf_plt pointer arithmetic

Kui-Feng Lee reported a crash on s390x triggered by the
dummy_st_ops/dummy_init_ptr_arg test [1]:

[<0000000000000002>] 0x2
[<00000000009d5cde>] bpf_struct_ops_test_run+0x156/0x250
[<000000000033145a>] __sys_bpf+0xa1a/0xd00
[<00000000003319dc>] __s390x_sys_bpf+0x44/0x50
[<0000000000c4382c>] __do_syscall+0x244/0x300
[<0000000000c59a40>] system_call+0x70/0x98

This is caused by GCC moving memcpy() after assignments in
bpf_jit_plt(), resulting in NULL pointers being written instead of
the return and the target addresses.

Looking at the GCC internals, the reordering is allowed because the
alias analysis thinks that the memcpy() destination and the assignments'
left-hand-sides are based on different objects: new_plt and
bpf_plt_ret/bpf_plt_target respectively, and therefore they cannot
alias.

This is in turn due to a violation of the C standard:

When two pointers are subtracted, both shall point to elements of the
same array object, or one past the last element of the array object
...

From the C's perspective, bpf_plt_ret and bpf_plt are distinct objects
and cannot be subtracted. In the practical terms, doing so confuses the
GCC's alias analysis.

The code was written this way in order to let the C side know a few
offsets defined in the assembly. While nice, this is by no means
necessary. Fix the noncompliance by hardcoding these offsets.

[1] htt...

Classification

CVE ID: CVE-2024-35917

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.05% (probability of being exploited)

EPSS Percentile: 17.83% (scored less or equal to compared to others)

EPSS Date: 2025-02-04 (when was this score calculated)

References

https://git.kernel.org/stable/c/c3062bdb859b6e2567e7f5c8cde20c0250bb130f
https://git.kernel.org/stable/c/d3d74e45a060d218fe4b0c9174f0a77517509d8e
https://git.kernel.org/stable/c/7ded842b356d151ece8ac4985940438e6d3998bb

Timeline