Determine the actual canary location

gef.py

@@ -2260,6 +2260,9 @@ def is_branch_taken(self, insn: Instruction) -> Tuple[bool, str]:
     def get_ra(self, insn: Instruction, frame: "gdb.Frame") -> Optional[int]:
         raise NotImplementedError
 
+    def canary_address(self) -> int:
+        raise NotImplementedError
+
     @classmethod
     def mprotect_asm(cls, addr: int, size: int, perm: Permission) -> str:
         raise NotImplementedError
@@ -2920,6 +2923,8 @@ def mprotect_asm(cls, addr: int, size: int, perm: Permission) -> str:
         ]
         return "; ".join(insns)
 
+    def canary_address(self) -> int:
+        return self.register("fs_base") + 0x28
 
 class PowerPC(Architecture):
     aliases = ("PowerPC", Elf.Abi.POWERPC, "PPC")
@@ -10439,7 +10444,6 @@ def reset_caches(self) -> None:
         self._os = None
         self._pid = None
         self._file = None
-        self._canary = None
         self._maps: Optional[pathlib.Path] = None
         self._root: Optional[pathlib.Path] = None
         return
@@ -10513,15 +10517,26 @@ def pagesize(self) -> int:
 
     @property
     def canary(self) -> Optional[Tuple[int, int]]:
-        """Returns a tuple of the canary address and value, read from the auxiliary vector."""
-        auxval = self.auxiliary_vector
-        if not auxval:
-            return None
-        canary_location = auxval["AT_RANDOM"]
-        canary = gef.memory.read_integer(canary_location)
-        canary &= ~0xFF
-        self._canary = (canary, canary_location)
-        return self._canary
+        """Return a tuple of the canary address and value, read from the canonical
+        location if supported by the architecture. Otherwise, read from the auxiliary
+        vector."""
+        try:
+            canary_location = gef.arch.canary_address()
+            canary = gef.memory.read_integer(canary_location)
+        except NotImplementedError:
+            # Fall back to `AT_RANDOM`, which is the original source
+            # of the canary value but not the canonical location
+            auxval = self.auxiliary_vector
+            if not auxval:
+                return None
+            canary_location = auxval["AT_RANDOM"]
+            canary = gef.memory.read_integer(canary_location)
+            warn("Canary was read from auxiliary vector")
+            # The canary is created by reading a word from `AT_RANDOM`
+            # and zeroing the lowest byte
+            canary &= ~0xFF
+
+        return canary, canary_location
 
     @property
     def maps(self) -> Optional[pathlib.Path]:

tests/commands/canary.py

@@ -5,7 +5,7 @@
 
 from tests.utils import gdb_start_silent_cmd, gdb_run_cmd, _target
 from tests.utils import GefUnitTestGeneric
-
+import platform
 
 class CanaryCommand(GefUnitTestGeneric):
     """`canary` command test module"""
@@ -16,3 +16,11 @@ def test_cmd_canary(self):
         res = gdb_start_silent_cmd("canary", target=_target("canary"))
         self.assertNoException(res)
         self.assertIn("The canary of process", res)
+        # On other platforms, we read the canary from the auxiliary vector -
+        # overwriting this value does not overwrite the in-use canary so it
+        # is not tested
+        if platform.machine() == "x86_64":
+            patch = r"pi gef.memory.write(gef.arch.canary_address(), b'\xef\xbe\xad\xde')"
+            res = gdb_start_silent_cmd(patch, target=_target("canary"), after=["canary"])
+            self.assertNoException(res)
+            self.assertIn("0xdeadbeef", res)