Subject: CVE-2024-35873: riscv: Fix vector state restore in rt_sigreturn()

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2024-35873 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.8 with commit c2a658d41924 and fixed in 6.8.5 with commit 5b16d904e910
	Issue introduced in 6.8 with commit c2a658d41924 and fixed in 6.9 with commit c27fa53b858b

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35873
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/riscv/kernel/signal.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/5b16d904e910183181b9d90efa957c787a8ac91b
	https://git.kernel.org/stable/c/c27fa53b858b4ee6552a719aa599c250cf98a586