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 618515e80bb28ad9855605fb160705b93188a4bf..c2a91e1c2bd2bb9b2724210c6758086e62fddf77 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.
+
 2016-06-21  Florian Weimer  <fweimer@redhat.com>
 
        [BZ #20284]

diff --git a/NEWS b/NEWS
index 923468dd2ea9e7852fdd1e78cb364e1582f8b2b3..a19f656448928073b5c0a79ec4b9308b11dfa84e 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,12 @@ using `glibc' in the "product" field.
 \f
 Version 2.22.1
 
+* 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)
+
 * A stack-based buffer overflow was found in libresolv when invoked from
   libnss_dns, allowing specially crafted DNS responses to seize control
   of execution flow in the DNS client.  The buffer overflow occurs in

diff --git a/sysdeps/unix/sysv/linux/arm/setcontext.S b/sysdeps/unix/sysv/linux/arm/setcontext.S
index 24c72945c8551b8e0ee32a11275978817237cb73..926b65af6f23134d53490487f2c51a40a979039d 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