Re: [PATCH 26/34] mm: implement new mprotect_key() system call
From: Michael Kerrisk (man-pages)
Date: Sat Dec 05 2015 - 01:51:04 EST
Dave,
On 12/04/2015 02:15 AM, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> mprotect_key() is just like mprotect, except it also takes a
> protection key as an argument. On systems that do not support
> protection keys, it still works, but requires that key=0.
> Otherwise it does exactly what mprotect does.
Is there a man page for this API?
Thanks,
Michael
> I expect it to get used like this, if you want to guarantee that
> any mapping you create can *never* be accessed without the right
> protection keys set up.
>
> pkey_deny_access(11); // random pkey
> int real_prot = PROT_READ|PROT_WRITE;
> ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
> ret = mprotect_key(ptr, PAGE_SIZE, real_prot, 11);
>
> This way, there is *no* window where the mapping is accessible
> since it was always either PROT_NONE or had a protection key set.
>
> We settled on 'unsigned long' for the type of the key here. We
> only need 4 bits on x86 today, but I figured that other
> architectures might need some more space.
>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: linux-api@xxxxxxxxxxxxxxx
> ---
>
> b/arch/x86/include/asm/mmu_context.h | 10 +++++++--
> b/include/linux/pkeys.h | 7 +++++-
> b/mm/Kconfig | 7 ++++++
> b/mm/mprotect.c | 36 +++++++++++++++++++++++++++++------
> 4 files changed, 51 insertions(+), 9 deletions(-)
>
> diff -puN arch/x86/include/asm/mmu_context.h~pkeys-85-mprotect_pkey arch/x86/include/asm/mmu_context.h
> --- a/arch/x86/include/asm/mmu_context.h~pkeys-85-mprotect_pkey 2015-12-03 16:21:30.181877894 -0800
> +++ b/arch/x86/include/asm/mmu_context.h 2015-12-03 16:21:30.190878302 -0800
> @@ -4,6 +4,7 @@
> #include <asm/desc.h>
> #include <linux/atomic.h>
> #include <linux/mm_types.h>
> +#include <linux/pkeys.h>
>
> #include <trace/events/tlb.h>
>
> @@ -243,10 +244,14 @@ static inline void arch_unmap(struct mm_
> mpx_notify_unmap(mm, vma, start, end);
> }
>
> +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> +/*
> + * If the config option is off, we get the generic version from
> + * include/linux/pkeys.h.
> + */
> static inline int vma_pkey(struct vm_area_struct *vma)
> {
> u16 pkey = 0;
> -#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> unsigned long vma_pkey_mask = VM_PKEY_BIT0 | VM_PKEY_BIT1 |
> VM_PKEY_BIT2 | VM_PKEY_BIT3;
> /*
> @@ -259,9 +264,10 @@ static inline int vma_pkey(struct vm_are
> */
> pkey = (vma->vm_flags >> vm_pkey_shift) &
> (vma_pkey_mask >> vm_pkey_shift);
> -#endif
> +
> return pkey;
> }
> +#endif
>
> static inline bool __pkru_allows_pkey(u16 pkey, bool write)
> {
> diff -puN include/linux/pkeys.h~pkeys-85-mprotect_pkey include/linux/pkeys.h
> --- a/include/linux/pkeys.h~pkeys-85-mprotect_pkey 2015-12-03 16:21:30.183877985 -0800
> +++ b/include/linux/pkeys.h 2015-12-03 16:21:30.190878302 -0800
> @@ -2,10 +2,10 @@
> #define _LINUX_PKEYS_H
>
> #include <linux/mm_types.h>
> -#include <asm/mmu_context.h>
>
> #ifdef CONFIG_ARCH_HAS_PKEYS
> #include <asm/pkeys.h>
> +#include <asm/mmu_context.h>
> #else /* ! CONFIG_ARCH_HAS_PKEYS */
>
> /*
> @@ -17,6 +17,11 @@ static inline bool arch_validate_pkey(in
> {
> return true;
> }
> +
> +static inline int vma_pkey(struct vm_area_struct *vma)
> +{
> + return 0;
> +}
> #endif /* ! CONFIG_ARCH_HAS_PKEYS */
>
> #endif /* _LINUX_PKEYS_H */
> diff -puN mm/Kconfig~pkeys-85-mprotect_pkey mm/Kconfig
> --- a/mm/Kconfig~pkeys-85-mprotect_pkey 2015-12-03 16:21:30.185878075 -0800
> +++ b/mm/Kconfig 2015-12-03 16:21:30.190878302 -0800
> @@ -673,3 +673,10 @@ config ARCH_USES_HIGH_VMA_FLAGS
> bool
> config ARCH_HAS_PKEYS
> bool
> +
> +config NR_PROTECTION_KEYS
> + int
> + # Everything supports a _single_ key, so allow folks to
> + # at least call APIs that take keys, but require that the
> + # key be 0.
> + default 1
> diff -puN mm/mprotect.c~pkeys-85-mprotect_pkey mm/mprotect.c
> --- a/mm/mprotect.c~pkeys-85-mprotect_pkey 2015-12-03 16:21:30.186878121 -0800
> +++ b/mm/mprotect.c 2015-12-03 16:21:30.191878347 -0800
> @@ -24,6 +24,7 @@
> #include <linux/migrate.h>
> #include <linux/perf_event.h>
> #include <linux/ksm.h>
> +#include <linux/pkeys.h>
> #include <asm/uaccess.h>
> #include <asm/pgtable.h>
> #include <asm/cacheflush.h>
> @@ -344,10 +345,13 @@ fail:
> return error;
> }
>
> -SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
> - unsigned long, prot)
> +/*
> + * pkey=-1 when doing a legacy mprotect()
> + */
> +static int do_mprotect_pkey(unsigned long start, size_t len,
> + unsigned long prot, int pkey)
> {
> - unsigned long vm_flags, nstart, end, tmp, reqprot;
> + unsigned long nstart, end, tmp, reqprot;
> struct vm_area_struct *vma, *prev;
> int error = -EINVAL;
> const int grows = prot & (PROT_GROWSDOWN|PROT_GROWSUP);
> @@ -373,8 +377,6 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
> if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
> prot |= PROT_EXEC;
>
> - vm_flags = calc_vm_prot_bits(prot, 0);
> -
> down_write(¤t->mm->mmap_sem);
>
> vma = find_vma(current->mm, start);
> @@ -407,7 +409,14 @@ SYSCALL_DEFINE3(mprotect, unsigned long,
>
> /* Here we know that vma->vm_start <= nstart < vma->vm_end. */
>
> - newflags = vm_flags;
> + /*
> + * If this is a vanilla, non-pkey mprotect, inherit the
> + * pkey from the VMA we are working on.
> + */
> + if (pkey == -1)
> + newflags = calc_vm_prot_bits(prot, vma_pkey(vma));
> + else
> + newflags = calc_vm_prot_bits(prot, pkey);
> newflags |= (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
>
> /* newflags >> 4 shift VM_MAY% in place of VM_% */
> @@ -443,3 +452,18 @@ out:
> up_write(¤t->mm->mmap_sem);
> return error;
> }
> +
> +SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
> + unsigned long, prot)
> +{
> + return do_mprotect_pkey(start, len, prot, -1);
> +}
> +
> +SYSCALL_DEFINE4(pkey_mprotect, unsigned long, start, size_t, len,
> + unsigned long, prot, int, pkey)
> +{
> + if (!arch_validate_pkey(pkey))
> + return -EINVAL;
> +
> + return do_mprotect_pkey(start, len, prot, pkey);
> +}
> _
> --
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--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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