tcg/ppc: Fix race in goto_tb implementation

Commit 20b6643 ("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 20b6643 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: 20b6643 ("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>
(cherry picked from commit 736a158)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

tcg/ppc/tcg-target.c.inc

@@ -2546,11 +2546,10 @@ 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.  */
@@ -2578,10 +2577,12 @@ 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;
     }