tcg/ppc: Fix race in goto_tb implementation

Commit 20b6643324 ("tcg/ppc: Reorg goto_tb implementation") modified
goto_tb to ensure only a single instruction was patched to prevent
incorrect behavior 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 relevant 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 initialize 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.

Fixes: 20b6643324 ("tcg/ppc: Reorg goto_tb implementation")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1726
Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Tested-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Co-developed-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Message-Id: <20230717093001.13167-1-jniethe5@gmail.com>

0 files changed, 0 insertions, 0 deletions