Inexplicable PROT_EXEC flag set on mmap callback
Kenneth Adam Miller
kennethadammiller at gmail.com
Sat Jan 16 13:33:32 EST 2016
The particular non-C binary that I'm using is rust with musl support, so
that I can statically compile the binary in order to eliminate all library
dependencies and then run it on a buildroot based linux.
On Sat, Jan 16, 2016 at 1:32 PM, Kenneth Adam Miller <
kennethadammiller at gmail.com> wrote:
> 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
>> > > > >>
>> > > > >>
>> > > > >
>> > >
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20160116/7e7dbc34/attachment.html
More information about the Kernelnewbies
mailing list