Re: [PATCH v23 28/28] mm: Introduce PROT_SHSTK for shadow stack

Kirill A. Shutemov
2021-03-22T11:27:09Z

On Tue, Mar 16, 2021 at 08:10:54AM -0700, Yu-cheng Yu wrote:
> There are three possible options to create a shadow stack allocation API:
> an arch_prctl, a new syscall, or adding PROT_SHSTK to mmap()/mprotect().
> Each has its advantages and compromises.
>
> An arch_prctl() is the least intrusive. However, the existing x86
> arch_prctl() takes only two parameters. Multiple parameters must be
> passed in a memory buffer. There is a proposal to pass more parameters in
> registers [1], but no active discussion on that.
>
> A new syscall minimizes compatibility issues and offers an extensible frame
> work to other architectures, but this will likely result in some overlap of
> mmap()/mprotect().
>
> The introduction of PROT_SHSTK to mmap()/mprotect() takes advantage of
> existing APIs. The x86-specific PROT_SHSTK is translated to VM_SHSTK and
> a shadow stack mapping is created without reinventing the wheel. There are
> potential pitfalls though. The most obvious one would be using this as a
> bypass to shadow stack protection. However, the attacker would have to get
> to the syscall first.
>
> [1] https://lore.kernel.org/lkml/20200828121624.108243-1-hjl.tools@gmail.com/
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> arch/x86/include/asm/mman.h | 57 +++++++++++++++++++++++++++++++-
> arch/x86/include/uapi/asm/mman.h | 1 +
> include/linux/mm.h | 1 +
> mm/mmap.c | 8 ++++-
> 4 files changed, 65 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/mman.h b/arch/x86/include/asm/mman.h
> index 629f6c81263a..bd94e30b5d34 100644
> --- a/arch/x86/include/asm/mman.h
> +++ b/arch/x86/include/asm/mman.h
> @@ -20,11 +20,66 @@
> ((vm_flags) & VM_PKEY_BIT2 ? _PAGE_PKEY_BIT2 : 0) | \
> ((vm_flags) & VM_PKEY_BIT3 ? _PAGE_PKEY_BIT3 : 0))
>
> -#define arch_calc_vm_prot_bits(prot, key) ( \
> +#define pkey_vm_prot_bits(prot, key) ( \
> ((key) & 0x1 ? VM_PKEY_BIT0 : 0) | \
> ((key) & 0x2 ? VM_PKEY_BIT1 : 0) | \
> ((key) & 0x4 ? VM_PKEY_BIT2 : 0) | \
> ((key) & 0x8 ? VM_PKEY_BIT3 : 0))
> +#else
> +#define pkey_vm_prot_bits(prot, key) (0)
> +#endif
> +
> +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
> + unsigned long pkey)
> +{
> + unsigned long vm_prot_bits = pkey_vm_prot_bits(prot, pkey);
> +
> + if (!(prot & PROT_WRITE) && (prot & PROT_SHSTK))
> + vm_prot_bits |= VM_SHSTK;
> +
> + return vm_prot_bits;
> +}
> +
> +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
> +
> +#ifdef CONFIG_X86_CET
> +static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
> +{
> + unsigned long valid = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM;
> +
> + if (prot & ~(valid | PROT_SHSTK))

Why PROT_SHSTK is not part of valid?

> + return false;
> +
> + if (prot & PROT_SHSTK) {
> + struct vm_area_struct *vma;
> +
> + if (!current->thread.cet.shstk_size)
> + return false;
> +
> + /*
> + * A shadow stack mapping is indirectly writable by only
> + * the CALL and WRUSS instructions, but not other write
> + * instructions). PROT_SHSTK and PROT_WRITE are mutually
> + * exclusive.
> + */
> + if (prot & PROT_WRITE)
> + return false;
> +
> + vma = find_vma(current->mm, addr);
> + if (!vma)
> + return false;

NAK.

This is racy. arch_validate_prot() called outside of mmap_lock and the vma
may be freed or modified under us.

do_mprotect_pkey() already calls find_vma() with the right locking. Maybe
re-strucure do_mprotect_pkey() to call arch_validate_prot() after
find_vma() and pass down the vma?

> +
> + /*
> + * Shadow stack cannot be backed by a file or shared.
> + */
> + if (vma->vm_file || (vma->vm_flags & VM_SHARED))
> + return false;
> + }
> +
> + return true;
> +}
> +
> +#define arch_validate_prot arch_validate_prot
> #endif
>
> #endif /* _ASM_X86_MMAN_H */
> diff --git a/arch/x86/include/uapi/asm/mman.h b/arch/x86/include/uapi/asm/mman.h
> index 3ce1923e6ed9..39bb7db344a6 100644
> --- a/arch/x86/include/uapi/asm/mman.h
> +++ b/arch/x86/include/uapi/asm/mman.h
> @@ -4,6 +4,7 @@
>
> #define MAP_32BIT 0x40 /* only give out 32bit addresses */
>
> +#define PROT_SHSTK 0x10 /* shadow stack pages */
>
> #include <asm-generic/mman.h>
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e178be052419..40c4b0fe7cc4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -342,6 +342,7 @@ extern unsigned int kobjsize(const void *objp);
>
> #if defined(CONFIG_X86)
> # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */
> +# define VM_ARCH_CLEAR VM_SHSTK
> #elif defined(CONFIG_PPC)
> # define VM_SAO VM_ARCH_1 /* Strong Access Ordering (powerpc) */
> #elif defined(CONFIG_PARISC)
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 99077171010b..934cb3cbe952 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1481,6 +1481,12 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> struct inode *inode = file_inode(file);
> unsigned long flags_mask;
>
> + /*
> + * Call stack cannot be backed by a file.
> + */
> + if (vm_flags & VM_SHSTK)
> + return -EINVAL;
> +
> if (!file_mmap_ok(file, inode, pgoff, len))
> return -EOVERFLOW;
>
> @@ -1545,7 +1551,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> } else {
> switch (flags & MAP_TYPE) {
> case MAP_SHARED:
> - if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
> + if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP|VM_SHSTK))
> return -EINVAL;
> /*
> * Ignore pgoff.
> --
> 2.21.0
>

--
Kirill A. Shutemov