[tip: x86/urgent] x86/debug: Only clear/set ->virtual_dr6 for userspace #DB

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     a195f3d4528a2f88d6f986f6b1101775ad4891cf
Gitweb:        https://git.kernel.org/tip/a195f3d4528a2f88d6f986f6b1101775ad4891cf
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 27 Oct 2020 10:15:06 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 27 Oct 2020 23:15:23 +01:00

x86/debug: Only clear/set ->virtual_dr6 for userspace #DB

The ->virtual_dr6 is the value used by ptrace_{get,set}_debugreg(6). A
kernel #DB clearing it could mean spurious malfunction of ptrace()
expectations.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Kyle Huey <me@xxxxxxxxxxxx> 
Link: https://lore.kernel.org/r/20201027093608.028952500@xxxxxxxxxxxxx

---
 arch/x86/kernel/traps.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index b5208aa..32b2d39 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -793,12 +793,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
 	set_debugreg(DR6_RESERVED, 6);
 	dr6 ^= DR6_RESERVED; /* Flip to positive polarity */
 
-	/*
-	 * Clear the virtual DR6 value, ptrace routines will set bits here for
-	 * things we want signals for.
-	 */
-	current->thread.virtual_dr6 = 0;
-
 	return dr6;
 }
 
@@ -943,6 +937,12 @@ 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.
+	 */
+	current->thread.virtual_dr6 = 0;
+
+	/*
 	 * The SDM says "The processor clears the BTF flag when it
 	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
 	 * TIF_BLOCKSTEP in sync with the hardware BTF flag.