Inexplicable PROT_EXEC flag set on mmap callback

Kenneth Adam Miller kennethadammiller at gmail.com
Sat Jan 16 13:32:51 EST 2016


Wait, are you assuming that I'm using the latest kernel? Because I'm using
3.14.56...

On Sat, Jan 16, 2016 at 1:31 PM, Mike Krinkin <krinkin.m.u at gmail.com> wrote:

> On Sat, Jan 16, 2016 at 01:16:42PM -0500, Kenneth Adam Miller wrote:
> > Ok, so you think that the format of the binary would influence the kernel
> > to change the permissions on the user's behalf? There's not much prose
> > explanation here, and I don't understand why the kernel would do
> something
> > like this.
>
> That personality falg was introduced here with quite a detail explanation
> (which i don't understand though):
> http://lwn.net/Articles/94068/
>
> > I just wanted to use a static binary to eliminate library
> > dependency issues between my host machine and the target machine. I had
> no
> > idea that settings like this would carry over to my task at hand.
>
> I compiled simple hello world with -static flag, and GNU_STACK in the
> binary
> has no executable flag set, so static has probably nothing to do with this.
>
> >
> > On Sat, Jan 16, 2016 at 1:08 PM, Mike Krinkin <krinkin.m.u at gmail.com>
> wrote:
> >
> > > On Sat, Jan 16, 2016 at 12:45:17PM -0500, Kenneth Adam Miller wrote:
> > > > I got the strace output of my non-C binary (I filtered the noise out
> of
> > > the
> > > > output for you):
> > > >
> > > > mmap(NULL, 8192, PROT_READ | PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
> -1,
> > > 0)
> > > >
> > > > I also have readelf -l output:
> > > >
> > > > Elf file type is EXEC (Executable file)
> > > > Entry point 0x401311
> > > > There are 7 program headers, starting at offset 64
> > > >
> > > > Program Headers:
> > > >   Type           Offset             VirtAddr           PhysAddr
> > > >                  FileSiz            MemSiz              Flags  Align
> > > >   LOAD           0x0000000000000000 0x0000000000400000
> 0x0000000000400000
> > > >                  0x00000000000db604 0x00000000000db604  R E    1000
> > > >   LOAD           0x00000000000dc1c0 0x00000000004dd1c0
> 0x00000000004dd1c0
> > > >                  0x0000000000006220 0x00000000000091dc  RW     1000
> > > >   NOTE           0x00000000000001c8 0x00000000004001c8
> 0x00000000004001c8
> > > >                  0x0000000000000024 0x0000000000000024  R      4
> > > >   GNU_EH_FRAME   0x00000000000d5680 0x00000000004d5680
> 0x00000000004d5680
> > > >                  0x0000000000005f84 0x0000000000005f84  R      4
> > > >   GNU_STACK      0x0000000000000000 0x0000000000000000
> 0x0000000000000000
> > > >                  0x0000000000000000 0x0000000000000000  RWE    0
> > >
> > > Well, probably this is a bit more relevant:
> > > http://lxr.free-electrons.com/source/mm/mmap.c#L1281
> > >
> > > As far as i can see, kernel sets READ_IMPLIES_EXEC flag here:
> > > http://lxr.free-electrons.com/source/fs/binfmt_elf.c#L844
> > >
> > > if executable_stack != EXSTACK_DISABLE_X, and executable_stack
> initialized
> > > here:
> > > http://lxr.free-electrons.com/source/fs/binfmt_elf.c#L781
> > >
> > > if GNU_STACK has an executable flag set (and i suppose, that RWE means,
> > > that
> > > in your case GNU_STACK indeed has exectuable flag set).
> > >
> > > It may be a reason, i'm not shure though. May be this can help:
> > > http://man7.org/linux/man-pages/man2/personality.2.html
> > >
> > >
> > > >   TLS            0x00000000000dc1c0 0x00000000004dd1c0
> 0x00000000004dd1c0
> > > >                  0x0000000000000100 0x0000000000000100  R      10
> > > >   GNU_RELRO      0x00000000000dc1c0 0x00000000004dd1c0
> 0x00000000004dd1c0
> > > >                  0x0000000000005e40 0x0000000000005e40  RW     20
> > > >
> > > >  Section to Segment mapping:
> > > >   Segment Sections...
> > > >    00     .note.gnu.build-id .init .text .fini .gcc_except_table
> .rodata
> > > > .debug_gdb_scripts .eh_frame .eh_frame_hdr
> > > >    01     .tdata .data.rel.ro.local .data.rel.ro .init_array .got
> > > .got.plt
> > > > .data .bss
> > > >    02     .note.gnu.build-id
> > > >    03     .eh_frame_hdr
> > > >    04
> > > >    05     .tdata
> > > >    06     .tdata .data.rel.ro.local .data.rel.ro .init_array .got
> > > .got.plt
> > > >
> > > > Some notes:
> > > >
> > > > As a test, I changed the non-C binary's target device file to
> /dev/zero,
> > > > and then I could see that the non-C mmap attempt would succeed just
> fine.
> > > >
> > > > After further verification and debugging based on guidance from
> another
> > > > forum, I have convinced that the vm_flags change must be occuring
> > > somewhere
> > > > in kernel land after control flow has left user land. Now I need to
> > > figure
> > > > out how to use a kernel debugger or kprobes to walk through the
> execution
> > > > of mmap callback delegation and see where the flags parameter is
> being
> > > > changed.
> > > >
> > > > I was pointed out to this:
> > > > http://lxr.free-electrons.com/source/mm/mmap.c#L1312
> > > >
> > > > But why would my vm_flags be changed by the kernel? And what can I
> do to
> > > > get this to stop? Why is the kernel changing the vm_flags for a non-C
> > > > binary using my device file, but not for either a C binary using my
> > > device
> > > > file or any type of binary that's not using my device file?
> > > >
> > > > On Thu, Jan 14, 2016 at 12:28 PM, Kenneth Adam Miller <
> > > > kennethadammiller at gmail.com> wrote:
> > > >
> > > > >
> > > > >
> > > > > On Thu, Jan 14, 2016 at 12:00 PM, Mike Krinkin <
> krinkin.m.u at gmail.com>
> > > > > wrote:
> > > > >
> > > > >> Hi, i have a couple of questions to clarify, if you don't mind
> > > > >>
> > > > >> On Thu, Jan 14, 2016 at 11:04:28AM -0500, Kenneth Adam Miller
> wrote:
> > > > >> > I have a custom drive and userland program pair that I'm using
> for a
> > > > >> very
> > > > >> > special use case at my workplace where we are mapping specific
> > > physical
> > > > >> > address ranges into userland memory with a mmap callback.
> Everything
> > > > >> works
> > > > >> > together well with a C userland program that calls into our
> driver's
> > > > >> ioctl
> > > > >> > and mmap definitions, but for our case we are using an
> alternative
> > > > >> systems
> > > > >> > language just for the userland program.
> > > > >>
> > > > >> So you have userland app written in C, and another not written in
> C?
> > > > >> The former works well while the latter doesn't, am i right?
> > > > >>
> > > > >
> > > > > Yes, the former works in so much as mmap completes successfully.
> I've
> > > > > verified that the
> > > > > parameters are identical in the non-C program. The issue of just
> using
> > > the
> > > > > C only program
> > > > > is that the actual implementation of interest is in the non-C
> program,
> > > and
> > > > > that's because
> > > > > that language facilitates other features that are *required* on our
> > > end.
> > > > >
> > > > >
> > > > >>
> > > > >> > That mmap call is failing (properly
> > > > >> > as we want) out from the driver's mmap implementation due to the
> > > fact
> > > > >> that
> > > > >> > the vm_flags have the VM_EXEC flag set. We do not want users to
> be
> > > able
> > > > >> to
> > > > >> > map the memory range as executable, so the driver should check
> for
> > > this
> > > > >> as
> > > > >> > it does. The issue is in the fact that somewhere between where
> mmap
> > > is
> > > > >> > called and when the parameters are given to the driver, the
> > > > >> vma->vm_flags
> > > > >> > are being set to 255. I've manually checked the values being
> given
> > > to
> > > > >> the
> > > > >> > mmap call in our non-C binary, and they are *equivalent* in
> value to
> > > > >> that
> > > > >> > of the C program.
> > > > >>
> > > > >> By "manually" do you mean strace? Could you show strace output for
> > > > >> both apps? And also could you show readelf -l output for both
> > > binaries?
> > > > >>
> > > > >
> > > > > By manually, I mean with a print call just before the mmap call in
> > > each of
> > > > > the
> > > > > programs. Right now, I'm working on getting a strace output, but I
> > > have to
> > > > > run that in qemu.
> > > > > To be able to run it in qemu in order to isolate the driver and all
> > > from
> > > > > my host, I have to build
> > > > > with buildroot. So I'll email that when I get it, but it'll be a
> while.
> > > > >
> > > > >
> > > > >>
> > > > >> >
> > > > >> > My question is, is there anything that can cause the
> vma->vm_flags
> > > to be
> > > > >> > changed in the trip between when the user land program calls
> mmap
> > > and
> > > > >> when
> > > > >> > control is delivered to the mmap callback?
> > > > >>
> > > > >> > _______________________________________________
> > > > >> > Kernelnewbies mailing list
> > > > >> > Kernelnewbies at kernelnewbies.org
> > > > >> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> > > > >>
> > > > >>
> > > > >
> > >
>
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