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SubjectRe: [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm
On Thu, Feb 21, 2019 at 03:44:34PM -0800, Rick Edgecombe wrote:
> From: Nadav Amit <namit@vmware.com>
>
> Prevent user watchpoints from mistakenly firing while the temporary mm
> is being used. As the addresses that of the temporary mm might overlap
> those of the user-process, this is necessary to prevent wrong signals
> or worse things from happening.
>
> Cc: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> arch/x86/include/asm/mmu_context.h | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index d684b954f3c0..0d6c72ece750 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -13,6 +13,7 @@
> #include <asm/tlbflush.h>
> #include <asm/paravirt.h>
> #include <asm/mpx.h>
> +#include <asm/debugreg.h>
>
> extern atomic64_t last_mm_ctx_id;
>
> @@ -358,6 +359,7 @@ static inline unsigned long __get_current_cr3_fast(void)
>
> typedef struct {
> struct mm_struct *prev;
> + unsigned short bp_enabled : 1;
> } temp_mm_state_t;
>
> /*
> @@ -380,6 +382,22 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
> lockdep_assert_irqs_disabled();
> state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
> switch_mm_irqs_off(NULL, mm, current);
> +
> + /*
> + * If breakpoints are enabled, disable them while the temporary mm is
> + * used. Userspace might set up watchpoints on addresses that are used
> + * in the temporary mm, which would lead to wrong signals being sent or
> + * crashes.
> + *
> + * Note that breakpoints are not disabled selectively, which also causes
> + * kernel breakpoints (e.g., perf's) to be disabled. This might be
> + * undesirable, but still seems reasonable as the code that runs in the
> + * temporary mm should be short.
> + */
> + state.bp_enabled = hw_breakpoint_active();

Pretty sure caching hw_breakpoint_active() is unnecessary. It queries a
per-cpu value, not hardware's DR7 register, and that same value is
consumed by hw_breakpoint_restore(). No idea if breakpoints can be
disabled while using a temp mm, but even if that can happen, there's no
need to restore breakpoints if they've all been disabled, i.e. if
hw_breakpoint_active() returns false in unuse_temporary_mm().

> + if (state.bp_enabled)
> + hw_breakpoint_disable();
> +
> return state;
> }
>
> @@ -387,6 +405,13 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev)
> {
> lockdep_assert_irqs_disabled();
> switch_mm_irqs_off(NULL, prev.prev, current);
> +
> + /*
> + * Restore the breakpoints if they were disabled before the temporary mm
> + * was loaded.
> + */
> + if (prev.bp_enabled)
> + hw_breakpoint_restore();
> }
>
> #endif /* _ASM_X86_MMU_CONTEXT_H */
> --
> 2.17.1
>

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