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; \ })