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Re: [PATCH 00/12] x86/CET: The last 12 patches to enable Intel CET

On 07/27/2018 04:53 PM, Florian Weimer wrote:
On 07/26/2018 01:16 PM, Florian Weimer wrote:
On 07/21/2018 04:20 PM, H.J. Lu wrote:
These are the last 12 patches to enable Intel CET.  Tested by

2. With --enable-cet and without --enable-cet for i686, x86-64 and x32
on non-CET x86-64 processors.
3. With --enable-cet for x86-64 and x32 on CET SDV using the CET kernel
from cet branch at:

When the shadow stack (SHSTK) is enabled, makecontext needs to allocate
a new shadow stack to go with the new stack allocated by the caller.
setcontext and swapcontext must properly handle the corresponding shadow
stack when the stack is switched.  Add more tests for user context
functions to provide more coverage for the shadow stack support.

Building glibc with --enable-cet currently breaks valgrind (even for current CPUs which do not support CET):

Mark is working on a valgrind fix.

This has now been fixed (in valgrind-3.13.0-22.fc29.x86_64), but I get this instead:

==22== Conditional jump or move depends on uninitialised value(s)
==22==    at 0x10DF43: _dl_process_cet_property_note (dl-prop.h:82)
==22==    by 0x10DF43: _dl_process_pt_note (dl-prop.h:137)
==22==    by 0x10E713: _dl_map_object_from_fd (dl-load.c:1170)
==22==    by 0x11120B: _dl_map_object (dl-load.c:2241)
==22==    by 0x10D92B: dl_main (rtld.c:1050)
==22==    by 0x121A6E: _dl_sysdep_start (dl-sysdep.c:253)
==22==    by 0x10A1D7: _dl_start_final (rtld.c:413)
==22==    by 0x10A1D7: _dl_start (rtld.c:520)
==22==    by 0x109117: ??? (in …/build-x86_64-redhat-linux/elf/
==22==    by 0x3: ???
==22==    by 0x1FFF000746: ???
==22==    by 0x1FFF000750: ???
==22==    by 0x1FFF00075F: ???
==22==    by 0x1FFF000770: ???
==22==  Uninitialised value was created by a stack allocation
==22==    at 0x10DE80: _dl_process_pt_note (dl-prop.h:111)

One potential cause could be that note traversal loop is buggy.

GDB says:

(gdb) bt
#0  0x000000000010df43 in _dl_process_cet_property_note (align=<optimized out>, size=48, note=0x1ffefff930, l=0x134160)
     at ../sysdeps/x86/dl-prop.h:82
#1  _dl_process_pt_note (l=l@entry=0x134160, ph=ph@entry=0x1fff000018, fd=fd@entry=3, fbp=fbp@entry=0x1ffefffe10)
     at ../sysdeps/x86/dl-prop.h:137
#2  0x000000000010e714 in _dl_map_object_from_fd (name=name@entry=0x1fff000721 "/usr/bin/true", origname=origname@entry=0x0,     fd=<optimized out>, fbp=fbp@entry=0x1ffefffe10, realname=<optimized out>, loader=loader@entry=0x0, l_type=<optimized out>,     mode=<optimized out>, stack_endp=<optimized out>, nsid=<optimized out>) at dl-load.c:1170 #3  0x000000000011120c in _dl_map_object (loader=loader@entry=0x0, name=0x1fff000721 "/usr/bin/true", type=type@entry=0,     trace_mode=trace_mode@entry=0, mode=mode@entry=536870912, nsid=nsid@entry=0) at dl-load.c:2241 #4  0x000000000010d92c in dl_main (phdr=<optimized out>, phnum=10, user_entry=0x1fff0003d8, auxv=<optimized out>) at rtld.c:1050 #5  0x0000000000121a6f in _dl_sysdep_start (start_argptr=start_argptr@entry=0x1fff0004b0, dl_main=dl_main@entry=0x10a610 <dl_main>)
     at ../elf/dl-sysdep.c:253
#6  0x000000000010a1d8 in _dl_start_final (arg=0x1fff0004b0) at rtld.c:413
#7  _dl_start (arg=0x1fff0004b0) at rtld.c:520
#8  0x0000000000109118 in _start ()

The problem seems to be this:

           unsigned int type = *(unsigned int *) ptr;
           unsigned int datasz = *(unsigned int *) (ptr + 4);

           ptr += 8;
           if ((ptr + datasz) > ptr_end)

We load datasz and then check for having encountered the last note.  I'm checking a fix.

The attached patch should fix this.  Only lightly tested.

Subject: [PATCH] x86: Fix out of bounds access in CET note parser

2018-07-27  Florian Weimer  <>

	* sysdeps/x86/dl-prop.h (_dl_process_cet_property_note): Fixed
	out-of-bounds access in note parser.

diff --git a/sysdeps/x86/dl-prop.h b/sysdeps/x86/dl-prop.h
index d56e20a6dc..9ee70ebc01 100644
--- a/sysdeps/x86/dl-prop.h
+++ b/sysdeps/x86/dl-prop.h
@@ -73,7 +73,7 @@ _dl_process_cet_property_note (struct link_map *l,
 	  unsigned char *ptr = (unsigned char *) (note + 1) + 4;
 	  unsigned char *ptr_end = ptr + note->n_descsz;
-	  while (1)
+	  while (ptr_end - ptr >= 8)
 	      unsigned int type = *(unsigned int *) ptr;
 	      unsigned int datasz = *(unsigned int *) (ptr + 4);

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