From cb05143bdf428f280a5d519c82abf196d7871c11 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 27 Oct 2020 19:33:30 +0100
Subject: [PATCH] x86/debug: Fix DR_STEP vs ptrace_get_debugreg(6)
Commit d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to
thread.virtual_dr6") changed the semantics of the variable from random
collection of bits, to exactly only those bits that ptrace() needs.
Unfortunately this lost DR_STEP for PTRACE_{BLOCK,SINGLE}STEP.
Furthermore, it turns out that userspace expects DR_STEP to be
unconditionally available, even for manual TF usage outside of
PTRACE_{BLOCK,SINGLE}_STEP.
Fixes: d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")
Reported-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/20201027183330.GM2628@hirez.programming.kicks-ass.net
---
arch/x86/kernel/traps.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 32b2d39272e03..e19df6cde35d1 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -937,10 +937,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
instrumentation_begin();
/*
- * Clear the virtual DR6 value, ptrace() routines will set bits here
- * for things it wants signals for.
+ * Start the virtual/ptrace DR6 value with just the DR_STEP mask
+ * of the real DR6. ptrace_triggered() will set the DR_TRAPn bits.
+ *
+ * Userspace expects DR_STEP to be visible in ptrace_get_debugreg(6)
+ * even if it is not the result of PTRACE_SINGLESTEP.
*/
- current->thread.virtual_dr6 = 0;
+ current->thread.virtual_dr6 = (dr6 & DR_STEP);
/*
* The SDM says "The processor clears the BTF flag when it
--
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