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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v7 3/7] x86: Temporary disable SMAP to legally access user pages in kernel mode
On 08/05/14 10:21, Feng Wu wrote:
> Use STAC/CLAC to temporarily disable SMAP to allow legal accesses to
> user pages in kernel mode
>
> STAC/CLAC is not needed for compat_create_bounce_frame, since in this
> chunk of code, it only accesses the pv guest's kernel stack, which is
> in ring 1 for 32-bit pv guests.
>
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> xen/arch/x86/traps.c | 12 ++++++++++++
> xen/arch/x86/usercopy.c | 6 ++++++
> xen/arch/x86/x86_64/entry.S | 2 ++
> xen/include/asm-x86/uaccess.h | 8 ++++++--
> xen/include/asm-x86/x86_64/system.h | 4 +++-
> 5 files changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
> index 1082270..c992df1 100644
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -3825,6 +3825,18 @@ unsigned long do_get_debugreg(int reg)
>
> void asm_domain_crash_synchronous(unsigned long addr)
> {
> + /*
> + * We need clear AC bit here because in entry.S AC is set
> + * by ASM_STAC to temporarily allow accesses to user pages
> + * which is prevented by SMAP by default.
> + *
> + * For some code paths, where this function is called, clac()
> + * is not needed, but adding clac() here instead of each place
> + * asm_domain_crash_synchronous() is called can reduce the code
> + * redundancy, and it is harmless as well.
> + */
> + clac();
> +
> if ( addr == 0 )
> addr = this_cpu(last_extable_addr);
>
> diff --git a/xen/arch/x86/usercopy.c b/xen/arch/x86/usercopy.c
> index b79202b..4cc78f5 100644
> --- a/xen/arch/x86/usercopy.c
> +++ b/xen/arch/x86/usercopy.c
> @@ -14,6 +14,7 @@ unsigned long __copy_to_user_ll(void __user *to, const void
> *from, unsigned n)
> {
> unsigned long __d0, __d1, __d2, __n = n;
>
> + stac();
> asm volatile (
> " cmp $"STR(2*BYTES_PER_LONG-1)",%0\n"
> " jbe 1f\n"
> @@ -42,6 +43,7 @@ unsigned long __copy_to_user_ll(void __user *to, const void
> *from, unsigned n)
> : "=&c" (__n), "=&D" (__d0), "=&S" (__d1), "=&r" (__d2)
> : "0" (__n), "1" (to), "2" (from), "3" (__n)
> : "memory" );
> + clac();
>
> return __n;
> }
> @@ -51,6 +53,7 @@ __copy_from_user_ll(void *to, const void __user *from,
> unsigned n)
> {
> unsigned long __d0, __d1, __d2, __n = n;
>
> + stac();
> asm volatile (
> " cmp $"STR(2*BYTES_PER_LONG-1)",%0\n"
> " jbe 1f\n"
> @@ -85,6 +88,7 @@ __copy_from_user_ll(void *to, const void __user *from,
> unsigned n)
> : "=&c" (__n), "=&D" (__d0), "=&S" (__d1), "=&r" (__d2)
> : "0" (__n), "1" (to), "2" (from), "3" (__n)
> : "memory" );
> + clac();
>
> return __n;
> }
> @@ -113,6 +117,7 @@ copy_to_user(void __user *to, const void *from, unsigned
> n)
> #define __do_clear_user(addr,size) \
> do { \
> long __d0; \
> + stac(); \
> __asm__ __volatile__( \
> "0: rep; stosl\n" \
> " movl %2,%0\n" \
> @@ -126,6 +131,7 @@ do {
> \
> _ASM_EXTABLE(1b,2b) \
> : "=&c"(size), "=&D" (__d0) \
> : "r"(size & 3), "0"(size / 4), "1"((long)addr), "a"(0));
> \
> + clac(); \
> } while (0)
>
> /**
> diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
> index ed7b96f..a81566e 100644
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -379,6 +379,7 @@ __UNLIKELY_END(create_bounce_frame_bad_sp)
> movb TRAPBOUNCE_flags(%rdx),%cl
> subq $40,%rsi
> movq UREGS_ss+8(%rsp),%rax
> + ASM_STAC
> .Lft2: movq %rax,32(%rsi) # SS
> movq UREGS_rsp+8(%rsp),%rax
> .Lft3: movq %rax,24(%rsi) # RSP
> @@ -423,6 +424,7 @@ UNLIKELY_END(bounce_failsafe)
> .Lft12: movq %rax,8(%rsi) # R11
> movq UREGS_rcx+8(%rsp),%rax
> .Lft13: movq %rax,(%rsi) # RCX
> + ASM_CLAC
> /* Rewrite our stack frame and return to guest-OS mode. */
> /* IA32 Ref. Vol. 3: TF, VM, RF and NT flags are cleared on trap. */
> /* Also clear AC: alignment checks shouldn't trigger in kernel mode.
> */
> diff --git a/xen/include/asm-x86/uaccess.h b/xen/include/asm-x86/uaccess.h
> index 88b4ba2..947470d 100644
> --- a/xen/include/asm-x86/uaccess.h
> +++ b/xen/include/asm-x86/uaccess.h
> @@ -146,6 +146,7 @@ struct __large_struct { unsigned long buf[100]; };
> * aliasing issues.
> */
> #define __put_user_asm(x, addr, err, itype, rtype, ltype, errret) \
> + stac(); \
> __asm__ __volatile__( \
> "1: mov"itype" %"rtype"1,%2\n" \
> "2:\n" \
> @@ -155,9 +156,11 @@ struct __large_struct { unsigned long buf[100]; };
> ".previous\n" \
> _ASM_EXTABLE(1b, 3b) \
> : "=r"(err) \
> - : ltype (x), "m"(__m(addr)), "i"(errret), "0"(err))
> + : ltype (x), "m"(__m(addr)), "i"(errret), "0"(err)); \
> + clac()
>
> #define __get_user_asm(x, addr, err, itype, rtype, ltype, errret) \
> + stac(); \
> __asm__ __volatile__( \
> "1: mov"itype" %2,%"rtype"1\n" \
> "2:\n" \
> @@ -168,7 +171,8 @@ struct __large_struct { unsigned long buf[100]; };
> ".previous\n" \
> _ASM_EXTABLE(1b, 3b) \
> : "=r"(err), ltype (x) \
> - : "m"(__m(addr)), "i"(errret), "0"(err))
> + : "m"(__m(addr)), "i"(errret), "0"(err)); \
> + clac()
>
> /**
> * __copy_to_user: - Copy a block of data into user space, with less checking
> diff --git a/xen/include/asm-x86/x86_64/system.h
> b/xen/include/asm-x86/x86_64/system.h
> index 20f038b..662813a 100644
> --- a/xen/include/asm-x86/x86_64/system.h
> +++ b/xen/include/asm-x86/x86_64/system.h
> @@ -12,6 +12,7 @@
> * is the same as the initial value of _o then _n is written to location _p.
> */
> #define __cmpxchg_user(_p,_o,_n,_isuff,_oppre,_regtype) \
> + stac(); \
> asm volatile ( \
> "1: lock; cmpxchg"_isuff" %"_oppre"2,%3\n" \
> "2:\n" \
> @@ -22,7 +23,8 @@
> _ASM_EXTABLE(1b, 3b) \
> : "=a" (_o), "=r" (_rc) \
> : _regtype (_n), "m" (*__xg((volatile void *)_p)), "0" (_o), "1" (0)
> \
> - : "memory");
> + : "memory"); \
> + clac()
>
> #define cmpxchg_user(_p,_o,_n) \
> ({ \
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