Re: [PATCH] tcg/ppc: Fix race in goto_tb implementation

[Anushree Mathur]

On 7/17/23 06:53, Jordan Niethe wrote:
> Commit 20b6643324 ("tcg/ppc: Reorg goto_tb implementation") modified
> goto_tb to ensure only a single instruction was patched to prevent
> incorrect behaviour if a thread was in the middle of multiple
> instructions when they were replaced. However this introduced a race
> between loading the jmp target into TCG_REG_TB and patching and
> executing the direct branch.
>
> The relevent part of the goto_tb implementation:
>
>      ld TCG_REG_TB, TARGET_ADDR_LOCATION(TCG_REG_TB)
>    patch_location:
>      mtctr TCG_REG_TB
>      bctr
>
> tb_target_set_jmp_target() will replace 'patch_location' with a direct
> branch if the target is in range. The direct branch now relies on
> TCG_REG_TB being set up correctly by the ld. Prior to this commit
> multiple instructions were patched in for the direct branch case; these
> instructions would initalise TCG_REG_TB to the same value as the branch
> target.
>
> Imagine the following sequence:
>
> 1) Thread A is executing the goto_tb sequence and loads the jmp
>     target into TCG_REG_TB.
>
> 2) Thread B updates the jmp target address and calls
>     tb_target_set_jmp_target(). This patches a new direct branch into the
>     goto_tb sequence.
>
> 3) Thread A executes the newly patched direct branch. The value in
>     TCG_REG_TB still contains the old jmp target.
>
> TCG_REG_TB MUST contain the translation block's tc.ptr. Execution will
> eventually crash after performing memory accesses generated from a
> faulty value in TCG_REG_TB.
>
> This presents as segfaults or illegal instruction exceptions.
>
> Do not revert commit 20b6643324 as it did fix a different race
> condition. Instead remove the direct branch optimization and always use
> indirect branches.
>
> The direct branch optimization can be re-added later with a race free
> sequence.
>
> Gitlab issue: https://gitlab.com/qemu-project/qemu/-/issues/1726
>
> Fixes: 20b6643324 ("tcg/ppc: Reorg goto_tb implementation")
>
> Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
> Co-developed-by: Benjamin Gray <bgray@linux.ibm.com>
> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
I tested with -smp 2/4/8/16 and it works fine with all.

Tested-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>

Thanks,

Anushree-Mathur

> ---
>   tcg/ppc/tcg-target.c.inc | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
> index 8d6899cf40..a7323f479b 100644
> --- a/tcg/ppc/tcg-target.c.inc
> +++ b/tcg/ppc/tcg-target.c.inc
> @@ -2533,11 +2533,10 @@ static void tcg_out_goto_tb(TCGContext *s, int which)
>           ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
>           tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
>       
> -        /* Direct branch will be patched by tb_target_set_jmp_target. */
> +        /* TODO: Use direct branches when possible. */
>           set_jmp_insn_offset(s, which);
>           tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
>
> -        /* When branch is out of range, fall through to indirect. */
>           tcg_out32(s, BCCTR | BO_ALWAYS);
>
>           /* For the unlinked case, need to reset TCG_REG_TB.  */
> @@ -2565,10 +2564,11 @@ void tb_target_set_jmp_target(const TranslationBlock 
> *tb, int n,
>       intptr_t diff = addr - jmp_rx;
>       tcg_insn_unit insn;
>
> +    if (USE_REG_TB)
> +        return;
> +
>       if (in_range_b(diff)) {
>           insn = B | (diff & 0x3fffffc);
> -    } else if (USE_REG_TB) {
> -        insn = MTSPR | RS(TCG_REG_TB) | CTR;
>       } else {
>           insn = NOP;
>       }