arm: mark __startcontext as .cantunwind (bug 20435)

__startcontext marks the bottom of the call stack of the contexts created
by makecontext.

(cherry picked from commit 9e2ff6c9cc54c0b4402b8d49e4abe7000fde7617)

Also includes the NEWS update, cherry-picked from commits
056dd72af83f5459ce6d545a49dea6dba7d635dc and
4d047efdbc55b0d68947cde682e5363d16a66294.

diff --git a/ChangeLog b/ChangeLog
index 0598caa614e37e40056adb9f96b7bba8120e3c58..178aab2bbc59c20b038ee676b844fc5ab96b48ee 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-08-15  Andreas Schwab  <schwab@suse.de>
+
+       [BZ #20435]
+       * sysdeps/unix/sysv/linux/arm/setcontext.S (__startcontext): Mark
+       as .cantunwind.
+
 2016-08-17  Florian Weimer  <fweimer@redhat.com>
 
        [BZ #20452]

diff --git a/NEWS b/NEWS
index 07f3fe8569de35a67cce2835eb2be39fae43e053..e03e9f1f3fed2d079fa190969448c5d043033540 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -22,6 +22,12 @@ Security related changes:
   flooded with crafted ICMP and UDP messages.  Reported by Aldy Hernandez'
   alloca plugin for GCC.  (CVE-2016-4429)
 
+* On ARM EABI (32-bit), generating a backtrace for execution contexts which
+  have been created with makecontext could fail to terminate due to a
+  missing .cantunwind annotation.  This has been observed to lead to a hang
+  (denial of service) in some Go applications compiled with gccgo.  Reported
+  by Andreas Schwab.  (CVE-2016-6323)
+
 The following bugs are resolved with this release:
 
   [19679] gcc-4.9.3 C++ exception handling broken due to unaligned stack

diff --git a/sysdeps/unix/sysv/linux/arm/setcontext.S b/sysdeps/unix/sysv/linux/arm/setcontext.S
index 603e508858b491c6973552bd680bf3fc927f4714..d1f168fece80fd55bc79946f849c6fab4ff43e1b 100644 (file)
--- a/sysdeps/unix/sysv/linux/arm/setcontext.S
+++ b/sysdeps/unix/sysv/linux/arm/setcontext.S
@@ -86,12 +86,19 @@ weak_alias(__setcontext, setcontext)
 
        /* Called when a makecontext() context returns.  Start the
           context in R4 or fall through to exit().  */
+       /* Unwind descriptors are looked up based on PC - 2, so we have to
+          make sure to mark the instruction preceding the __startcontext
+          label as .cantunwind.  */
+       .fnstart
+       .cantunwind
+       nop
 ENTRY(__startcontext)
        movs    r0, r4
        bne     PLTJMP(__setcontext)
 
        @ New context was 0 - exit
        b       PLTJMP(HIDDEN_JUMPTARGET(exit))
+       .fnend
 END(__startcontext)
 
 #ifdef PIC