[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH 22/22] x86/mm: zero stack on stack switch or reset


  • To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 29 Jul 2024 16:40:24 +0100
  • Autocrypt: addr=andrew.cooper3@xxxxxxxxxx; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA==
  • Cc: alejandro.vallejo@xxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>
  • Delivery-date: Mon, 29 Jul 2024 15:40:37 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 26/07/2024 4:22 pm, Roger Pau Monne wrote:
> With the stack mapped on a per-CPU basis there's no risk of other CPUs being
> able to read the stack contents, but vCPUs running on the current pCPU could
> read stack rubble from operations of previous vCPUs.
>
> The #DF stack is not zeroed because handling of #DF results in a panic.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/include/asm/current.h | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/include/asm/current.h 
> b/xen/arch/x86/include/asm/current.h
> index 75b9a341f814..02b4118b03ef 100644
> --- a/xen/arch/x86/include/asm/current.h
> +++ b/xen/arch/x86/include/asm/current.h
> @@ -177,6 +177,14 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
>  # define SHADOW_STACK_WORK ""
>  #endif
>  
> +#define ZERO_STACK                                              \
> +    "test %[stk_size], %[stk_size];"                            \
> +    "jz .L_skip_zeroing.%=;"                                    \
> +    "std;"                                                      \
> +    "rep stosb;"                                                \
> +    "cld;"                                                      \
> +    ".L_skip_zeroing.%=:"
> +
>  #if __GNUC__ >= 9
>  # define ssaj_has_attr_noreturn(fn) __builtin_has_attribute(fn, __noreturn__)
>  #else
> @@ -187,10 +195,24 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
>  #define switch_stack_and_jump(fn, instr, constr)                        \
>      ({                                                                  \
>          unsigned int tmp;                                               \
> +        bool zero_stack = current->domain->arch.asi;                    \
>          BUILD_BUG_ON(!ssaj_has_attr_noreturn(fn));                      \
> +        ASSERT(IS_ALIGNED((unsigned long)guest_cpu_user_regs() -        \
> +                          PRIMARY_STACK_SIZE +                          \
> +                          sizeof(struct cpu_info), PAGE_SIZE));         \
> +        if ( zero_stack )                                               \
> +        {                                                               \
> +            unsigned long stack_top = get_stack_bottom() &              \
> +                                      ~(STACK_SIZE - 1);                \
> +                                                                        \
> +            clear_page((void *)stack_top + IST_MCE * PAGE_SIZE);        \
> +            clear_page((void *)stack_top + IST_NMI * PAGE_SIZE);        \
> +            clear_page((void *)stack_top + IST_DB  * PAGE_SIZE);        \
> +        }                                                               \
>          __asm__ __volatile__ (                                          \
>              SHADOW_STACK_WORK                                           \
>              "mov %[stk], %%rsp;"                                        \
> +            ZERO_STACK                                                  \
>              CHECK_FOR_LIVEPATCH_WORK                                    \
>              instr "[fun]"                                               \
>              : [val] "=&r" (tmp),                                        \
> @@ -201,7 +223,13 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
>                ((PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8),               \
>                [stack_mask] "i" (STACK_SIZE - 1),                        \
>                _ASM_BUGFRAME_INFO(BUGFRAME_bug, __LINE__,                \
> -                                 __FILE__, NULL)                        \
> +                                 __FILE__, NULL),                       \
> +              /* For stack zeroing. */                                  \
> +              "D" ((void *)guest_cpu_user_regs() - 1),                  \
> +              [stk_size] "c"                                            \
> +              (zero_stack ? PRIMARY_STACK_SIZE - sizeof(struct cpu_info)\
> +                          : 0),                                         \
> +              "a" (0)                                                   \
>              : "memory" );                                               \
>          unreachable();                                                  \
>      })

This looks very expensive.

For starters, switch_stack_and_jump() is used twice in a typical context
switch; once in the schedule tail, and again out of hvm_do_resume().

Furthermore, #MC happen never (to many many significant figures), #DB
happens never for HVM guests (but does happen for PV), and NMIs are
either ~never, or 2Hz which is far less often than the 30ms default
timeslice.

So, the overwhelming majority of the time, those 3 calls to clear_page()
will be re-zeroing blocks of zeroes.

This can probably be avoided by making use of ist_exit (held in %r12) to
only zero an IST stack when leaving it.  This leaves the IRET frame able
to be recovered, but with e.g. RFDS, you can do that irrespective, and
it's not terribly sensitive.


What about shadow stacks?  You're not zeroing those, and while they're
less sensitive than the data stack, there ought to be some reasoning
about them.

~Andrew



 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.