[PATCH v5 0/2] x86/fred: Prevent immediate repeat of single step trap on return from SIGTRAP handler

["Xin Li (Intel)" <xin@zytor.com>]

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-xin@zytor.com/
[2] https://lore.kernel.org/lkml/20250530230707.2528916-1-xin@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-xin@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: stable@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