From 736a1588c104e9995c1831df33554df1f1def8b8 Mon Sep 17 00:00:00 2001 From: Jordan Niethe Date: Mon, 17 Jul 2023 19:30:01 +1000 Subject: [PATCH] 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 Tested-by: Anushree Mathur Tested-by: Michael Tokarev Reviewed-by: Richard Henderson Co-developed-by: Benjamin Gray Signed-off-by: Jordan Niethe Signed-off-by: Benjamin Gray Message-Id: <20230717093001.13167-1-jniethe5@gmail.com> --- tcg/ppc/tcg-target.c.inc | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc index c866f2c997..511e14b180 100644 --- a/tcg/ppc/tcg-target.c.inc +++ b/tcg/ppc/tcg-target.c.inc @@ -2496,11 +2496,10 @@ 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. */ @@ -2528,10 +2527,12 @@ 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; }