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

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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@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kyle Huey <me@kylehuey.com> 
Link: https://lore.kernel.org/r/20201027093608.028952500@infradead.org
This commit is contained in:
Peter Zijlstra 2020-10-27 10:15:06 +01:00 committed by Thomas Gleixner
parent 2a9baf5ad4
commit a195f3d452
1 changed files with 6 additions and 6 deletions
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12
arch/x86/kernel/traps.c
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@ -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;
}
@ -942,6 +936,12 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
irqentry_enter_from_user_mode(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