Inexplicable PROT_EXEC flag set on mmap callback
Mike Krinkin
krinkin.m.u at gmail.com
Sat Jan 16 13:31:52 EST 2016
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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