The following commit has been merged into the x86/urgent branch of tip: Commit-ID: 2a9baf5ad4884108b3c6d56a50e8105ccf8a4ee7 Gitweb: https://git.kernel.org/tip/2a9baf5ad4884108b3c6d56a50e8105ccf8a4ee7 Author: Peter Zijlstra <pet...@infradead.org> AuthorDate: Tue, 27 Oct 2020 10:15:05 +01:00 Committer: Thomas Gleixner <t...@linutronix.de> CommitterDate: Tue, 27 Oct 2020 23:15:23 +01:00
x86/debug: Fix BTF handling The SDM states that #DB clears DEBUGCTLMSR_BTF, this means that when the bit is set for userspace (TIF_BLOCKSTEP) and a kernel #DB happens first, the BTF bit meant for userspace execution is lost. Have the kernel #DB handler restore the BTF bit when it was requested for userspace. Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Tested-by: Kyle Huey <m...@kylehuey.com> Link: https://lore.kernel.org/r/20201027093607.956147...@infradead.org --- arch/x86/kernel/traps.c | 28 +++++++++++++++++++++------- 1 file changed, 21 insertions(+), 7 deletions(-) diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 3c70fb3..b5208aa 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -799,13 +799,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void) */ 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. - */ - clear_thread_flag(TIF_BLOCKSTEP); - return dr6; } @@ -873,6 +866,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs, */ WARN_ON_ONCE(user_mode(regs)); + if (test_thread_flag(TIF_BLOCKSTEP)) { + /* + * The SDM says "The processor clears the BTF flag when it + * generates a debug exception." but PTRACE_BLOCKSTEP requested + * it for userspace, but we just took a kernel #DB, so re-set + * BTF. + */ + unsigned long debugctl; + + rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl); + debugctl |= DEBUGCTLMSR_BTF; + wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl); + } + /* * Catch SYSENTER with TF set and clear DR_STEP. If this hit a * watchpoint at the same time then that will still be handled. @@ -936,6 +943,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs, instrumentation_begin(); /* + * 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. + */ + clear_thread_flag(TIF_BLOCKSTEP); + + /* * If dr6 has no reason to give us about the origin of this trap, * then it's very likely the result of an icebp/int01 trap. * User wants a sigtrap for that.
