CVE-2024-35917 s390/bpf: Fix bpf_plt pointer arithmetic
A pointer arithmetic error in the s390x BPF JIT compiler causes NULL pointer dereferences, leading to system crashes.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-35917?
CVE-2024-35917 is a medium-severity vulnerability in Linux kernel, published on 19 May 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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] https://lore.kernel.org/bpf/[email protected]/
Weakness: CWE-476
Which versions are affected by CVE-2024-35917?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-35917?
Apply the vendor's patch.
Is CVE-2024-35917 being exploited?
There is no known exploitation of CVE-2024-35917 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-35917, DEBIAN-CVE-2024-35917, GHSA-6p8g-x822-g299, UBUNTU-CVE-2024-35917.