[PATCH v4 09/11] gdb: Implement amd64 linux shadow stack support for inferior calls.

[Christina Schimpe <christina.schimpe@intel.com>]

This patch enables inferior calls to support Intel's Control-Flow
Enforcement Technology (CET), which provides the shadow stack feature
for the x86 architecture.
Following the restriction of the linux kernel, enable inferior calls
for amd64 only.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
---
 gdb/amd64-linux-tdep.c                        | 64 +++++++++++++++++++
 gdb/doc/gdb.texinfo                           | 29 +++++++++
 .../gdb.arch/amd64-shadow-stack-cmds.exp      | 55 +++++++++++++++-
 3 files changed, 147 insertions(+), 1 deletion(-)

diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index 9436f0b190c..d847248659a 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1931,6 +1931,68 @@ amd64_linux_shadow_stack_element_size_aligned (gdbarch *gdbarch)
   return (binfo->bits_per_word / binfo->bits_per_byte);
 }
 
+/* Read the shadow stack pointer register and return its value, if
+   possible.  */
+
+static std::optional<CORE_ADDR>
+amd64_linux_get_shadow_stack_pointer (gdbarch *gdbarch, regcache *regcache)
+{
+  const i386_gdbarch_tdep *tdep = gdbarch_tdep<i386_gdbarch_tdep> (gdbarch);
+
+  if (tdep == nullptr || tdep->ssp_regnum < 0)
+    return {};
+
+  CORE_ADDR ssp;
+  if (regcache_raw_read_unsigned (regcache, tdep->ssp_regnum, &ssp)
+      != REG_VALID)
+    return {};
+
+  /* Starting with v6.6., the Linux kernel supports CET shadow stack.
+     Dependent on the target the ssp register can be invalid or nullptr
+     when shadow stack is supported by HW and the linux kernel but not
+     enabled for the current thread.  */
+  if (ssp == 0x0)
+    return {};
+
+  return ssp;
+}
+
+/* If shadow stack is enabled, push the address NEW_ADDR on the shadow
+   stack and increment the shadow stack pointer accordingly.  */
+
+static void
+amd64_linux_shadow_stack_push (gdbarch *gdbarch, CORE_ADDR new_addr,
+			       regcache *regcache)
+{
+  std::optional<CORE_ADDR> ssp
+    = amd64_linux_get_shadow_stack_pointer (gdbarch, regcache);
+  if (!ssp.has_value ())
+    return;
+
+  /* The shadow stack grows downwards.  To push addresses on the stack,
+     we need to decrement SSP.    */
+  const int element_size
+    = amd64_linux_shadow_stack_element_size_aligned (gdbarch);
+  const CORE_ADDR new_ssp = *ssp - element_size;
+
+  /* Starting with v6.6., the Linux kernel supports CET shadow stack.
+     Using /proc/PID/smaps we can only check if NEW_SSP points to shadow
+     stack memory.  If it doesn't, we assume the stack is full.  */
+  std::pair<CORE_ADDR, CORE_ADDR> memrange;
+  if (!linux_address_in_shadow_stack_mem_range (new_ssp, &memrange))
+    error (_("No space left on the shadow stack."));
+
+  /* On x86 there can be a shadow stack token at bit 63.  For x32, the
+     address size is only 32 bit.  Thus, we must use ELEMENT_SIZE (and
+     not gdbarch_addr_bit) to determine the width of the address to be
+     written.  */
+  const bfd_endian byte_order = gdbarch_byte_order (gdbarch);
+  write_memory_unsigned_integer (new_ssp, element_size, byte_order,
+				 (ULONGEST) new_addr);
+
+  i386_gdbarch_tdep *tdep = gdbarch_tdep<i386_gdbarch_tdep> (gdbarch);
+  regcache_raw_write_unsigned (regcache, tdep->ssp_regnum, new_ssp);
+}
 
 /* Implement shadow stack pointer unwinding. For each new shadow stack
    pointer check if its address is still in the shadow stack memory range.
@@ -2057,6 +2119,8 @@ amd64_linux_init_abi_common(struct gdbarch_info info, struct gdbarch *gdbarch,
 
   set_gdbarch_remove_non_address_bits_watchpoint
     (gdbarch, amd64_linux_remove_non_address_bits_watchpoint);
+
+  set_gdbarch_shadow_stack_push (gdbarch, amd64_linux_shadow_stack_push);
   dwarf2_frame_set_init_reg (gdbarch, amd64_init_reg);
 }
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 0ae09f09c88..cf152bd1e6f 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -27033,6 +27033,35 @@ registers
 
 @end itemize
 
+@subsubsection Intel Control-Flow Enforcement Technology.
+@cindex Intel Control-Flow Enforcement Technology.
+
+The @dfn{Intel Control-Flow Enforcement Technology} (@acronym{Intel CET})
+provides two capabilities to defend against ``Return-oriented Programming''
+and ``call/jmp-oriented programming'' style control-flow attacks:
+
+@itemize @bullet
+@item Shadow Stack:
+A shadow stack is a second stack for a program.  It holds the return
+addresses pushed by the call instruction.  The @code{RET} instruction pops the
+return addresses from both call and shadow stack.  If the return addresses from
+the two stacks do not match, the processor signals a control protection
+exception.
+@item Indirect Branch Tracking (IBT):
+When IBT is enabled, the CPU implements a state machine that tracks indirect
+@code{JMP} and @code{CALL} instructions.  The state machine can be either IDLE
+or WAIT_FOR_ENDBRANCH.  In WAIT_FOR_ENDBRANCH state the next instruction in
+the program stream must be an @code{ENDBR} instruction, otherwise the
+processor signals a control protection exception.
+@end itemize
+
+Impact on Call/Print:
+Inferior calls in @value{GDBN} reset the current PC to the beginning of the
+function that is called.  No call instruction is executed, but the @code{RET}
+instruction actually is.  To avoid a control protection exception due to the
+missing return address on the shadow stack, @value{GDBN} pushes the new return
+address to the shadow stack and updates the shadow stack pointer.
+
 @node Alpha
 @subsection Alpha
 
diff --git a/gdb/testsuite/gdb.arch/amd64-shadow-stack-cmds.exp b/gdb/testsuite/gdb.arch/amd64-shadow-stack-cmds.exp
index 17f32ce3964..96f83678f39 100644
--- a/gdb/testsuite/gdb.arch/amd64-shadow-stack-cmds.exp
+++ b/gdb/testsuite/gdb.arch/amd64-shadow-stack-cmds.exp
@@ -13,12 +13,29 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-# Test shadow stack enabling for frame level update and the return command.
+# Test shadow stack enabling for frame level update, the return and the
+# call command.
+# As potential CET violations often only occur after resuming normal
+# execution, test normal program continuation after each return or call
+# commands.
 
 require allow_ssp_tests
 
 standard_testfile amd64-shadow-stack.c
 
+proc restart_and_run_infcall_call2 {} {
+    global binfile
+    clean_restart ${binfile}
+    if { ![runto_main] } {
+	return -1
+    }
+    set inside_infcall_str "The program being debugged stopped while in a function called from GDB"
+    gdb_breakpoint [ gdb_get_line_number "break call2" ]
+    gdb_continue_to_breakpoint "break call2" ".*break call2.*"
+    gdb_test "call (int) call2()" \
+	"Breakpoint \[0-9\]*, call2.*$inside_infcall_str.*"
+}
+
 save_vars { ::env(GLIBC_TUNABLES) } {
 
     append_environment GLIBC_TUNABLES "glibc.cpu.hwcaps" "SHSTK"
@@ -33,6 +50,42 @@ save_vars { ::env(GLIBC_TUNABLES) } {
 	return -1
     }
 
+    with_test_prefix "test inferior call and continue" {
+	gdb_breakpoint [ gdb_get_line_number "break call1" ]
+	gdb_continue_to_breakpoint "break call1" ".*break call1.*"
+
+	gdb_test "call (int) call2()" "= 42"
+
+	gdb_continue_to_end
+    }
+
+    with_test_prefix "test return inside an inferior call" {
+	restart_and_run_infcall_call2
+
+	gdb_test "return" "\#0.*call2.*" \
+	    "Test shadow stack return inside an inferior call" \
+	    "Make.*return now\\? \\(y or n\\) " "y"
+
+	gdb_continue_to_end
+    }
+
+    with_test_prefix "test return 'above' an inferior call" {
+	restart_and_run_infcall_call2
+
+	gdb_test "frame 2" "call2 ().*" "move to frame 'above' inferior call"
+
+	gdb_test "return" "\#0.*call1.*" \
+	    "Test shadow stack return 'above' an inferior call" \
+	    "Make.*return now\\? \\(y or n\\) " "y"
+
+	gdb_continue_to_end
+    }
+
+    clean_restart ${binfile}
+    if { ![runto_main] } {
+	return -1
+    }
+
     set call1_line [ gdb_get_line_number "break call1" ]
     set call2_line [ gdb_get_line_number "break call2" ]
 
-- 
2.34.1

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