[PATCH] riscv: Fix vector state restore in rt_sigreturn()

[Andy Chiu]

On Wed, Apr 3, 2024 at 3:27 PM Björn Töpel <bjorn at kernel.org> wrote:
>
> From: Björn Töpel <bjorn at rivosinc.com>
>
> 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.
>
> Link: https://github.com/riscv/riscv-isa-manual/releases/download/riscv-isa-release-8abdb41-2024-03-26/unpriv-isa-asciidoc.pdf # [1]
> Reported-by: Charlie Jenkins <charlie at rivosinc.com>
> Reported-by: Vineet Gupta <vgupta at kernel.org>
> Fixes: c2a658d41924 ("riscv: lib: vectorize copy_to_user/copy_from_user")
> Signed-off-by: Björn Töpel <bjorn at rivosinc.com>

Thanks for the findings!

Reviewed-by: Andy Chiu <andy.chiu at sifive.com>

> ---
>  arch/riscv/kernel/signal.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
> index 501e66debf69..5a2edd7f027e 100644
> --- a/arch/riscv/kernel/signal.c
> +++ b/arch/riscv/kernel/signal.c
> @@ -119,6 +119,13 @@ static long __restore_v_state(struct pt_regs *regs, void __user *sc_vec)
>         struct __sc_riscv_v_state __user *state = sc_vec;
>         void __user *datap;
>
> +       /*
> +        * Mark the vstate as clean prior performing the actual copy,
> +        * to avoid getting the vstate incorrectly clobbered by the
> +        *  discarded vector state.
> +        */
> +       riscv_v_vstate_set_restore(current, regs);
> +
>         /* Copy everything of __sc_riscv_v_state except datap. */
>         err = __copy_from_user(&current->thread.vstate, &state->v_state,
>                                offsetof(struct __riscv_v_ext_state, datap));
> @@ -133,13 +140,7 @@ static long __restore_v_state(struct pt_regs *regs, void __user *sc_vec)
>          * Copy the whole vector content from user space datap. Use
>          * copy_from_user to prevent information leak.
>          */
> -       err = copy_from_user(current->thread.vstate.datap, datap, riscv_v_vsize);
> -       if (unlikely(err))
> -               return err;
> -
> -       riscv_v_vstate_set_restore(current, regs);
> -
> -       return err;
> +       return copy_from_user(current->thread.vstate.datap, datap, riscv_v_vsize);
>  }
>  #else
>  #define save_v_state(task, regs) (0)
>
> base-commit: 7115ff4a8bfed3b9294bad2e111744e6abeadf1a
> --
> 2.40.1
>