[PATCH 1/2] arm64: Support execute-only permissions with Enhanced PAN
Dave Martin
Dave.Martin at arm.com
Thu Nov 19 13:52:05 EST 2020
On Thu, Nov 19, 2020 at 01:39:52PM +0000, Vladimir Murzin wrote:
> Enhanced Privileged Access Never (EPAN) allows Privileged Access Never
> to be used with Execute-only mappings.
>
> Absence of such support was a reason for 24cecc377463 ("arm64: Revert
> support for execute-only user mappings"). Thus now it can be revisited
> and re-enabled.
>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Signed-off-by: Vladimir Murzin <vladimir.murzin at arm.com>
> ---
> arch/arm64/Kconfig | 17 +++++++++++++++++
> arch/arm64/include/asm/cpucaps.h | 5 +++--
> arch/arm64/include/asm/pgtable-prot.h | 5 +++--
> arch/arm64/include/asm/pgtable.h | 14 +++++++++++++-
> arch/arm64/include/asm/sysreg.h | 1 +
> arch/arm64/kernel/cpufeature.c | 21 +++++++++++++++++++++
> arch/arm64/mm/fault.c | 3 +++
> 7 files changed, 61 insertions(+), 5 deletions(-)
>
[...]
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 1ee9400..b93222e 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -467,6 +467,9 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
> if (faulthandler_disabled() || !mm)
> goto no_context;
>
> + if (cpus_have_const_cap(ARM64_HAS_EPAN))
> + vm_flags &= ~VM_EXEC;
> +
IIUC, this would be telling __do_page_fault() that the access would have
succeeded with any kind of permissions except for write access, which
doesn't seem right.
Also, isn't vm_flags just overwritten by the code after the hunk?
The logic in __do_page_fault() looks like might not have been written
with the assumption that there might be more than a single set bit in
vm_flags.
But I'm not familiar with this code, and might be totally
misunderstanding what's going on here...
> if (user_mode(regs))
> mm_flags |= FAULT_FLAG_USER;
[...]
Cheers
---Dave
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