TLDRVuln - Cybersecurity Vulnerabilities Made Simple

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.