linux-kernel - Re: [PATCH v23 28/28] mm: Introduce PROT

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Date:   Mon, 22 Mar 2021 14:27:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v23 28/28] mm: Introduce PROT_SHSTK for shadow stack

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@...el.com>
> Reviewed-by: Kees Cook <keescook@...omium.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