Re: [v5 PATCH] arm64: mm: force write fault for atomic RMW instructions

[Yang Shi]

On 7/9/24 11:35 AM, Catalin Marinas wrote:
> On Tue, Jul 09, 2024 at 10:56:55AM -0700, Yang Shi wrote:
> > On 7/4/24 3:03 AM, Catalin Marinas wrote:
> > > I haven't figured out what the +24% case is in there, it seems pretty
> > > large.
> > I think I ran the test much more iterations and I didn't see such outlier
> > anymore.
> That's good, thanks for confirming.
>
> > > What you haven't benchmarked (I think) is the case where the instruction
> > > is in an exec-only mapping. The subsequent instruction read will fault
> > > and it adds to the overhead. Currently exec-only mappings are not
> > > widespread but I heard some people planning to move in this direction as
> > > a default build configuration.
> > I tested exec-only on QEMU tcg, but I don't have a hardware supported EPAN.
> > I don't think performance benchmark on QEMU tcg makes sense since it is
> > quite slow, such small overhead is unlikely measurable on it.
> Yeah, benchmarking under qemu is pointless. I think you can remove some
> of the ARM64_HAS_EPAN checks (or replaced them with ARM64_HAS_PAN) just
> for testing. For security reason, we removed this behaviour in commit
> 24cecc377463 ("arm64: Revert support for execute-only user mappings")
> but it's good enough for testing. This should give you PROT_EXEC-only
> mappings on your hardware.

Thanks for the suggestion. IIUC, I still can emulate exec-only even 
though hardware doesn't support EPAN? So it means reading exec-only area 
in kernel still can trigger fault, right?

And 24cecc377463 ("arm64: Revert support for execute-only user 
mappings") can't be reverted cleanly by git revert, so I did it manually 
as below.

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 6a8b71917e3b..0bdedd415e56 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -573,8 +573,8 @@ static int __kprobes do_page_fault(unsigned long 
far, unsigned long esr,
                /* Write implies read */
                vm_flags |= VM_WRITE;
                /* If EPAN is absent then exec implies read */
-               if (!alternative_has_cap_unlikely(ARM64_HAS_EPAN))
-                       vm_flags |= VM_EXEC;
+               //if (!alternative_has_cap_unlikely(ARM64_HAS_EPAN))
+               //      vm_flags |= VM_EXEC;
        }

        if (is_ttbr0_addr(addr) && is_el1_permission_fault(addr, esr, 
regs)) {
diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 642bdf908b22..d30265d424e4 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -19,7 +19,7 @@ static pgprot_t protection_map[16] __ro_after_init = {
        [VM_WRITE]                                      = PAGE_READONLY,
        [VM_WRITE | VM_READ]                            = PAGE_READONLY,
        /* PAGE_EXECONLY if Enhanced PAN */
-       [VM_EXEC]                                       = 
PAGE_READONLY_EXEC,
+       [VM_EXEC]                                       = PAGE_EXECONLY,
        [VM_EXEC | VM_READ]                             = 
PAGE_READONLY_EXEC,
        [VM_EXEC | VM_WRITE]                            = 
PAGE_READONLY_EXEC,
        [VM_EXEC | VM_WRITE | VM_READ]                  = 
PAGE_READONLY_EXEC,


Is it correct? Just wanted to make sure I did the right thing before 
running test.

>