| Message ID | CAPcyv4iBHTahi66deUT1iSPpA2W2wjYZSaiUYUjpdK10d5Rt7g@mail.gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, Jan 19, 2018 at 10:58:44PM -0800, Dan Williams wrote: > On Thu, Jan 18, 2018 at 4:01 PM, Dan Williams <dan.j.williams@intel.com> wrote: > > Changes since v3 [1] > > * Drop 'ifence_array_ptr' and associated compile-time + run-time > > switching and just use the masking approach all the time. > > > > * Convert 'get_user' to use pointer sanitization via masking rather than > > lfence. '__get_user' and associated paths still rely on > > lfence. (Linus) > > > > "Basically, the rule is trivial: find all 'stac' users, and use > > address masking if those users already integrate the limit > > check, and lfence they don't." > > > > * At syscall entry sanitize the syscall number under speculation > > to remove a user controlled pointer de-reference in kernel > > space. (Linus) > > > > * Fix a raw lfence in the kvm code (added for v4.15-rc8) to use > > 'array_ptr'. > > > > * Propose 'array_idx' as a way to sanitize user input that is > > later used as an array index, but where the validation is > > happening in a different code block than the array reference. > > (Christian). > > > > * Fix grammar in speculation.txt (Kees) > > > > --- > > > > Quoting Mark's original RFC: > > > > "Recently, Google Project Zero discovered several classes of attack > > against speculative execution. One of these, known as variant-1, allows > > explicit bounds checks to be bypassed under speculation, providing an > > arbitrary read gadget. Further details can be found on the GPZ blog [2] > > and the Documentation patch in this series." > > > > A precondition of using this attack on the kernel is to get a user > > controlled pointer de-referenced (under speculation) in privileged code. > > The primary source of user controlled pointers in the kernel is the > > arguments passed to 'get_user' and '__get_user'. An example of other > > user controlled pointers are user-controlled array / pointer offsets. > > > > Better tooling is needed to find more arrays / pointers with user > > controlled indices / offsets that can be converted to use 'array_ptr' or > > 'array_idx'. A few are included in this set, and these are not expected > > to be complete. That said, the 'get_user' protections raise the bar on > > finding a vulnerable gadget in the kernel. > > > > These patches are also available via the 'nospec-v4' git branch here: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4 > > I've pushed out a nospec-v4.1 with the below minor cleanup, a fixup of > the changelog for "kvm, x86: fix spectre-v1 mitigation", and added > Paolo's ack. > > git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4.1 > > diff --git a/include/linux/nospec.h b/include/linux/nospec.h > index 8af35be1869e..b8a9222e34d1 100644 > --- a/include/linux/nospec.h > +++ b/include/linux/nospec.h > @@ -37,7 +37,7 @@ static inline unsigned long array_ptr_mask(unsigned > long idx, unsigned long sz) > unsigned long _i = (idx); \ > unsigned long _mask = array_ptr_mask(_i, (sz)); \ > \ > - __u._ptr = _arr + (_i & _mask); \ > + __u._ptr = _arr + _i; \ > __u._bit &= _mask; \ > __u._ptr; \ hmm. I'm not sure it's the right thing to do, since the macro is forcing cpu to speculate subsequent load from null instead of valid pointer. As Linus said: " So that __u._bit masking wasn't masking the pointer, it was masking the value that was *read* from the pointer, so that you could know that an invalid access returned 0/NULL, not just the first value in the array. " imo just return _arr + (_i & _mask); is enough. No need for union games. The cpu will speculate the load from _arr[0] if _i is out of bounds which is the same as if user passed _i == 0 which would have passed bounds check anyway, so I don't see any data leak from populating cache with _arr[0] data. In-bounds access can do that just as well without any speculation.
On Sat, Jan 20, 2018 at 8:56 AM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote: > On Fri, Jan 19, 2018 at 10:58:44PM -0800, Dan Williams wrote: >> On Thu, Jan 18, 2018 at 4:01 PM, Dan Williams <dan.j.williams@intel.com> wrote: >> > Changes since v3 [1] >> > * Drop 'ifence_array_ptr' and associated compile-time + run-time >> > switching and just use the masking approach all the time. >> > >> > * Convert 'get_user' to use pointer sanitization via masking rather than >> > lfence. '__get_user' and associated paths still rely on >> > lfence. (Linus) >> > >> > "Basically, the rule is trivial: find all 'stac' users, and use >> > address masking if those users already integrate the limit >> > check, and lfence they don't." >> > >> > * At syscall entry sanitize the syscall number under speculation >> > to remove a user controlled pointer de-reference in kernel >> > space. (Linus) >> > >> > * Fix a raw lfence in the kvm code (added for v4.15-rc8) to use >> > 'array_ptr'. >> > >> > * Propose 'array_idx' as a way to sanitize user input that is >> > later used as an array index, but where the validation is >> > happening in a different code block than the array reference. >> > (Christian). >> > >> > * Fix grammar in speculation.txt (Kees) >> > >> > --- >> > >> > Quoting Mark's original RFC: >> > >> > "Recently, Google Project Zero discovered several classes of attack >> > against speculative execution. One of these, known as variant-1, allows >> > explicit bounds checks to be bypassed under speculation, providing an >> > arbitrary read gadget. Further details can be found on the GPZ blog [2] >> > and the Documentation patch in this series." >> > >> > A precondition of using this attack on the kernel is to get a user >> > controlled pointer de-referenced (under speculation) in privileged code. >> > The primary source of user controlled pointers in the kernel is the >> > arguments passed to 'get_user' and '__get_user'. An example of other >> > user controlled pointers are user-controlled array / pointer offsets. >> > >> > Better tooling is needed to find more arrays / pointers with user >> > controlled indices / offsets that can be converted to use 'array_ptr' or >> > 'array_idx'. A few are included in this set, and these are not expected >> > to be complete. That said, the 'get_user' protections raise the bar on >> > finding a vulnerable gadget in the kernel. >> > >> > These patches are also available via the 'nospec-v4' git branch here: >> > >> > git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4 >> >> I've pushed out a nospec-v4.1 with the below minor cleanup, a fixup of >> the changelog for "kvm, x86: fix spectre-v1 mitigation", and added >> Paolo's ack. >> >> git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec-v4.1 >> >> diff --git a/include/linux/nospec.h b/include/linux/nospec.h >> index 8af35be1869e..b8a9222e34d1 100644 >> --- a/include/linux/nospec.h >> +++ b/include/linux/nospec.h >> @@ -37,7 +37,7 @@ static inline unsigned long array_ptr_mask(unsigned >> long idx, unsigned long sz) >> unsigned long _i = (idx); \ >> unsigned long _mask = array_ptr_mask(_i, (sz)); \ >> \ >> - __u._ptr = _arr + (_i & _mask); \ >> + __u._ptr = _arr + _i; \ >> __u._bit &= _mask; \ >> __u._ptr; \ > > hmm. I'm not sure it's the right thing to do, since the macro > is forcing cpu to speculate subsequent load from null instead > of valid pointer. > As Linus said: " > So that __u._bit masking wasn't masking > the pointer, it was masking the value that was *read* from the > pointer, so that you could know that an invalid access returned > 0/NULL, not just the first value in the array. > " > imo just > return _arr + (_i & _mask); > is enough. No need for union games. > The cpu will speculate the load from _arr[0] if _i is out of bounds > which is the same as if user passed _i == 0 which would have passed > bounds check anyway, so I don't see any data leak from populating > cache with _arr[0] data. In-bounds access can do that just as well > without any speculation. scratch that. It's array_ptr, not array_access. The code will do if (!ptr) later, so yeah this api is fine.
diff --git a/include/linux/nospec.h b/include/linux/nospec.h index 8af35be1869e..b8a9222e34d1 100644 --- a/include/linux/nospec.h +++ b/include/linux/nospec.h @@ -37,7 +37,7 @@ static inline unsigned long array_ptr_mask(unsigned long idx, unsigned long sz) unsigned long _i = (idx); \ unsigned long _mask = array_ptr_mask(_i, (sz)); \ \ - __u._ptr = _arr + (_i & _mask); \ + __u._ptr = _arr + _i; \ __u._bit &= _mask; \ __u._ptr; \