CVE-2025-38433 riscv: fix runtime constant support for nommu kernels
A logic error in RISC-V nommu kernel patching causes invalid pointers and kernel panics, requiring a patch to prevent denial of service.
- 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-2025-38433?
CVE-2025-38433 is a medium-severity vulnerability in Linux kernel, published on 25 Jul 2025. 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:
riscv: fix runtime constant support for nommu kernels
the `__runtime_fixup_32` function does not handle the case where `val` is zero correctly (as might occur when patching a nommu kernel and referring to a physical address below the 4GiB boundary whose upper 32 bits are all zero) because nothing in the existing logic prevents the code from taking the `else` branch of both nop-checks and emitting two `nop` instructions.
This leaves random garbage in the register that is supposed to receive the upper 32 bits of the pointer instead of zero that when combined with the value for the lower 32 bits yields an invalid pointer and causes a kernel panic when that pointer is eventually accessed.
The author clearly considered the fact that if the `lui` is converted into a `nop` that the second instruction needs to be adjusted to become an `li` instead of an `addi`, hence introducing the `addi_insn_mask` variable, but didn't follow that logic through fully to the case where the `else` branch executes. To fix it just adjust the logic to ensure that the second `else` branch is not taken if the first instruction will be patched to a `nop`.
Weakness: CWE-476
Which versions are affected by CVE-2025-38433?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-38433?
Apply the vendor's patch.
Is CVE-2025-38433 being exploited?
There is no known exploitation of CVE-2025-38433 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-38433, GHSA-w922-jv62-78r5, UBUNTU-CVE-2025-38433.