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. 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.
On Sat, Jan 16, 2016 at 1:08 PM, Mike Krinkin <[email protected]> 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 < > > [email protected]> wrote: > > > > > > > > > > > On Thu, Jan 14, 2016 at 12:00 PM, Mike Krinkin <[email protected]> > > > 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 > > >> > [email protected] > > >> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > >> > > >> > > > >
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