Re: [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching
From: Andy Lutomirski <luto@kernel.org>
Date: 2019-02-11 22:47:58
Also in:
linux-integrity, linux-mm, lkml
On Mon, Feb 11, 2019 at 11:18 AM Nadav Amit [off-list ref] wrote:
quoted
On Feb 11, 2019, at 11:07 AM, Andy Lutomirski [off-list ref] wrote: I'm certainly amenable to other solutions, but this one does seem the least messy. I looked at my old patch, and it doesn't do what you want. I'd suggest you just add a percpu variable like cpu_dr7 and rig up some accessors so that it stays up to date. Then you can skip the dr7 writes if there are no watchpoints set. Also, EFI is probably a less interesting example than rare_write. With rare_write, especially the dynamically allocated variants that people keep coming up with, we'll need a swath of address space fully as large as the vmalloc area. and getting *that* right while still using the kernel address range might be more of a mess than we really want to deal with.As long as you feel comfortable with this solution, I’m fine with it. Here is what I have (untested). I prefer to save/restore all the DRs, because IIRC DR6 indications are updated even if breakpoints are disabled (in DR7). And anyhow, that is the standard interface.
Seems reasonable, but:
quoted hunk ↗ jump to hunk
-- >8 -- From: Nadav Amit <redacted> Date: Mon, 11 Feb 2019 03:07:08 -0800 Subject: [PATCH] mm: save DRs when loading temporary mm Signed-off-by: Nadav Amit <redacted> --- arch/x86/include/asm/mmu_context.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index d684b954f3c0..4f92ec3df149 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 +382,15 @@ 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 - they do not belong and might cause wrong signals or crashes. + */
Maybe clarify this? Add some mention that the specific problem is that user code could set a watchpoint on an address that is also used in the temporary mm. Arguably we should not disable *kernel* breakpoints a la perf, but that seems like quite a minor issue, at least as long as use_temporary_mm() doesn't get wider use. But a comment that this also disables perf breakpoints and that this could be undesirable might be in order as well.