IDT event delivery has a debug hole in which it does not generate #DB upon returning to userspace before the first userspace instruction is executed if the Trap Flag (TF) is set.
FRED closes this hole by introducing a software event flag, i.e., bit 17 of the augmented SS: if the bit is set and ERETU would result in RFLAGS.TF = 1, a single-step trap will be pending upon completion of ERETU. However I overlooked properly setting and clearing the bit in different situations. Thus when FRED is enabled, if the Trap Flag (TF) is set without an external debugger attached, it can lead to an infinite loop in the SIGTRAP handler. To avoid this, the software event flag in the augmented SS must be cleared, ensuring that no single-step trap remains pending when ERETU completes. This patch set combines the fix [1] and its corresponding selftest [2] (requested by Dave Hansen) into one patch set. [1] https://lore.kernel.org/lkml/20250523050153.3308237-1-...@zytor.com/ [2] https://lore.kernel.org/lkml/20250530230707.2528916-1-...@zytor.com/ This patch set is based on tip/x86/urgent branch as of today. Link to v4 of this patch set: https://lore.kernel.org/lkml/20250605181020.590459-1-...@zytor.com/ Changes in v5: *) Accurately rephrase the shortlog (hpa). *) Do "sub $-128, %rsp" rather than "add $128, %rsp", which is more efficient in code size (hpa). *) Add TB from Sohil. *) Add Cc: sta...@vger.kernel.org to all patches. Xin Li (Intel) (2): x86/fred/signal: Prevent immediate repeat of single step trap on return from SIGTRAP handler selftests/x86: Add a test to detect infinite sigtrap handler loop arch/x86/include/asm/sighandling.h | 22 +++++ arch/x86/kernel/signal_32.c | 4 + arch/x86/kernel/signal_64.c | 4 + tools/testing/selftests/x86/Makefile | 2 +- tools/testing/selftests/x86/sigtrap_loop.c | 98 ++++++++++++++++++++++ 5 files changed, 129 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/x86/sigtrap_loop.c base-commit: dd2922dcfaa3296846265e113309e5f7f138839f -- 2.49.0
