Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 19 Dec 2016 15:37:45 -0800 | Subject | Re: [PATCH] selftests/x86: Add a selftest for SYSRET to noncanonical addresses |
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On Mon, Dec 19, 2016 at 3:30 PM, Kirill A. Shutemov <kirill@shutemov.name> wrote: > On Mon, Dec 19, 2016 at 11:12:42AM -0800, Andy Lutomirski wrote: >> SYSRET to a noncanonical address will blow up on Intel CPUs. Linux >> needs to prevent this from happening in two major cases, and the >> criteria will become more complicated when support for larger virtual >> address spaces is added. >> >> A fast-path SYSCALL will fallthrough to the following instruction >> using SYSRET without any particular checking. To prevent fallthrough >> to a noncanonical address, Linux prevents the highest canonical page >> from being mapped. This test case checks a variety of possible maximum >> addresses to make sure that either we can't map code there or that >> SYSCALL fallthrough works. >> >> A slow-path system call can return anywhere. Linux needs to make sure >> that, if the return address is non-canonical, it won't use SYSRET. >> This test cases causes sigreturn() to return to a variety of addresses >> (with RCX == RIP) and makes sure that nothing explodes. >> >> Some of this code comes from Kirill Shutemov. >> >> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> >> Signed-off-by: Andy Lutomirski <luto@kernel.org> > > Thanks a lot. > > That's what I've got with 5-level paging:
[...]
Looks good.
>> + err(1, "mremap to 0x%p", new_address); > > "0x" is redundant for %p. > >> + } else { >> + printf("[OK]\tmremap to 0x%p failed\n", new_address); > > Ditto.
Will fix.
>> + test_sigreturn_to((1UL<<i)); > > Redundant parenthesis?
Indeed.
> >> + >> + clearhandler(SIGUSR1); >> + >> + sethandler(SIGSEGV, sigsegv_for_fallthrough, 0); >> + >> + /* This should execute on all kernels. */ >> + test_syscall_fallthrough_to((1UL << 47) - PAGE_SIZE); >> + >> + /* Make sure that we didn't screw up the mremap logic. */ >> + test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE); >> + >> + /* These are the interesting cases. */ >> + for (int i = 47; i < 64; i++) >> + test_syscall_fallthrough_to((1UL<<i)); > > Ditto. > > Also, "(1UL << i) - PAGE_SIZE" is interesting too. TASK_SIZE for 5-level > paging would be (1UL << 56) - PAGE_SIZE. I would be better to catch both > corner cases.
There's not much scope for error, though -- (1UL << 56) - PAGE_SIZE isn't really any different from any other address. I all add it, but I'm not sure I see any way that the kernel could plausible get it wrong. I guess it's comforting to see the boundary, though.
--Andy
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