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    SubjectRe: sudo x86info -a => kernel BUG at mm/usercopy.c:78!
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    On 30.03.2017 23:01, Dave Jones wrote:
    > On Thu, Mar 30, 2017 at 12:52:31PM -0700, Kees Cook wrote:
    > > On Thu, Mar 30, 2017 at 12:41 PM, Dave Jones <davej@codemonkey.org.uk> wrote:
    > > > On Thu, Mar 30, 2017 at 09:45:26AM -0700, Kees Cook wrote:
    > > > > On Wed, Mar 29, 2017 at 11:44 PM, Tommi Rantala
    > > > > <tommi.t.rantala@nokia.com> wrote:
    > > > > > Hi,
    > > > > >
    > > > > > Running:
    > > > > >
    > > > > > $ sudo x86info -a
    > > > > >
    > > > > > On this HP ZBook 15 G3 laptop kills the x86info process with segfault and
    > > > > > produces the following kernel BUG.
    > > > > >
    > > > > > $ git describe
    > > > > > v4.11-rc4-40-gfe82203
    > > > > >
    > > > > > It is also reproducible with the fedora kernel: 4.9.14-200.fc25.x86_64
    > > > > >
    > > > > > Full dmesg output here: https://pastebin.com/raw/Kur2mpZq
    > > > > >
    > > > > > [ 51.418954] usercopy: kernel memory exposure attempt detected from
    > > > > > ffff880000090000 (dma-kmalloc-256) (4096 bytes)
    > > > >
    > > > > This seems like a real exposure: the copy is attempting to read 4096
    > > > > bytes from a 256 byte object.
    > > >
    > > > The code[1] is doing a 4k read from /dev/mem in the range 0x90000 -> 0xa0000
    > > > According to arch/x86/mm/init.c:devmem_is_allowed, that's still valid..
    > > >
    > > > Note that the printk is using the direct mapping address. Is that what's
    > > > being passed down to devmem_is_allowed now ? If so, that's probably what broke.
    > >
    > > So this is attempting to read physical memory 0x90000 -> 0xa0000, but
    > > that's somehow resolving to a virtual address that is claimed by
    > > dma-kmalloc?? I'm confused how that's happening...
    >
    > The only thing that I can think of would be a rogue ptr in the bios
    > table, but that seems unlikely. Tommi, can you put strace of x86info -mp somewhere?
    > That will confirm/deny whether we're at least asking the kernel to do sane things.

    Indeed the bug happens when reading from /dev/mem:

    https://pastebin.com/raw/ZEJGQP1X

    # strace -f -y x86info -mp
    [...]
    open("/dev/mem", O_RDONLY) = 3</dev/mem>
    lseek(3</dev/mem>, 1038, SEEK_SET) = 1038
    read(3</dev/mem>, "\300\235", 2) = 2
    lseek(3</dev/mem>, 646144, SEEK_SET) = 646144
    read(3</dev/mem>,
    "\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
    1024) = 1024
    lseek(3</dev/mem>, 1043, SEEK_SET) = 1043
    read(3</dev/mem>, "w\2", 2) = 2
    lseek(3</dev/mem>, 645120, SEEK_SET) = 645120
    read(3</dev/mem>,
    "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
    1024) = 1024
    lseek(3</dev/mem>, 654336, SEEK_SET) = 654336
    read(3</dev/mem>,
    "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
    1024) = 1024
    lseek(3</dev/mem>, 983040, SEEK_SET) = 983040
    read(3</dev/mem>,
    "IFE$\245S\0\0\1\0\0\0\0\360y\0\0\360\220\260\30\237{=\23\10\17\0000\276\17\0"...,
    65536) = 65536
    lseek(3</dev/mem>, 917504, SEEK_SET) = 917504
    read(3</dev/mem>,
    "\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377"...,
    65536) = 65536
    lseek(3</dev/mem>, 524288, SEEK_SET) = 524288
    read(3</dev/mem>, <unfinished ...>) = ?
    +++ killed by SIGSEGV +++

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