CVE-2024-35873

In the Linux kernel, the following vulnerability has been resolved: riscv:
Fix vector state restore in rt_sigreturn() The RISC-V Vector specification
states in “Appendix D: Calling Convention for Vector State” [1] that
“Executing a system call causes all caller-saved vector registers (v0-v31,
vl, vtype) and vstart to become unspecified.”. In the RISC-V kernel this is
called “discarding the vstate”. Returning from a signal handler via the
rt_sigreturn() syscall, vector discard is also performed. However, this is
not an issue since the vector state should be restored from the sigcontext,
and therefore not care about the vector discard. The “live state” is the
actual vector register in the running context, and the “vstate” is the
vector state of the task. A dirty live state, means that the vstate and
live state are not in synch. When vectorized user_from_copy() was
introduced, an bug sneaked in at the restoration code, related to the
discard of the live state. An example when this go wrong: 1. A userland
application is executing vector code 2. The application receives a signal,
and the signal handler is entered. 3. The application returns from the
signal handler, using the rt_sigreturn() syscall. 4. The live vector state
is discarded upon entering the rt_sigreturn(), and the live state is marked
as “dirty”, indicating that the live state need to be synchronized with the
current vstate. 5. rt_sigreturn() restores the vstate, except the Vector
registers, from the sigcontext 6. rt_sigreturn() restores the Vector
registers, from the sigcontext, and now the vectorized user_from_copy() is
used. The dirty live state from the discard is saved to the vstate, making
the vstate corrupt. 7. rt_sigreturn() returns to the application, which
crashes due to corrupted vstate. Note that the vectorized user_from_copy()
is invoked depending on the value of CONFIG_RISCV_ISA_V_UCOPY_THRESHOLD.
Default is 768, which means that vlen has to be larger than 128b for this
bug to trigger. The fix is simply to mark the live state as non-dirty/clean
prior performing the vstate restore.