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

[Michael Tokarev]

17.07.2023 04:23, 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

I confirm this fixes the issue hit by debian as well, - 30 runs in
a row already with this patch and counting, while before it failed
almost reliable on first try, sometimes on 2nd.

Tested-by: Michael Tokarev <mjt@tls.msk.ru>

(this is max I can do, as I don't know tcg at all :) )

Thank you very much for the fix!

/mjt

> @@ -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);