Update SafeStack TODO in the safestack.cc

[ksvladimir]

Summary

This patch clarifies the TODO note at the top of safestack.cc and brings it more in sync with what we (the CPI team) actually plan to work on in the future.

[ksvladimir]

Updated to include the cfe-commits list as a subscriber.

[pcc]

LGTM

compiler-rt/trunk/lib/safestack/safestack.cc

 #include <pthread.h>
 #include <stddef.h>
 #include <sys/resource.h>
 #include <sys/user.h>
 
 #include "interception/interception.h"
 #include "sanitizer_common/sanitizer_common.h"
 
-// TODO: The runtime library does not currently protect the safe stack. The
-// protection of the (safe) stack can be provided by two alternative features
-// that requires C library support:
-//
-// 1) Protection via hardware segmentation on x32 architectures: the (safe)
-// stack segment (implicitly accessed via the %ss segment register) can be
-// separated from the data segment (implicitly accessed via the %ds segment
-// register). Dereferencing a pointer to the safe segment would result in a
-// segmentation fault.
-//
-// 2) Protection via information hiding on 64 bit architectures: the location of
-// the safe stack can be randomized through secure mechanisms, and the leakage
-// of the stack pointer can be prevented. Currently, libc can leak the stack
-// pointer in several ways (e.g. in longjmp, signal handling, user-level context
-// switching related functions, etc.). These can be fixed in libc and in other
-// low-level libraries, by either eliminating the escaping/dumping of the stack
-// pointer (i.e., %rsp) when that's possible, or by using encryption/PTR_MANGLE
-// (XOR-ing the dumped stack pointer with another secret we control and protect
-// better). (This is already done for setjmp in glibc.) Furthermore, a static
-// machine code level verifier can be ran after code generation to make sure
-// that the stack pointer is never written to memory, or if it is, its written
-// on the safe stack.
-//
-// Finally, while the Unsafe Stack pointer is currently stored in a thread local
-// variable, with libc support it could be stored in the TCB (thread control
-// block) as well, eliminating another level of indirection. Alternatively,
-// dedicating a separate register for storing it would also be possible.
+// TODO: The runtime library does not currently protect the safe stack beyond
+// relying on the system-enforced ASLR. The protection of the (safe) stack can
+// be provided by three alternative features:
+//
+// 1) Protection via hardware segmentation on x86-32 and some x86-64
+// architectures: the (safe) stack segment (implicitly accessed via the %ss
+// segment register) can be separated from the data segment (implicitly
+// accessed via the %ds segment register). Dereferencing a pointer to the safe
+// segment would result in a segmentation fault.
+//
+// 2) Protection via software fault isolation: memory writes that are not meant
+// to access the safe stack can be prevented from doing so through runtime
+// instrumentation. One way to do it is to allocate the safe stack(s) in the
+// upper half of the userspace and bitmask the corresponding upper bit of the
+// memory addresses of memory writes that are not meant to access the safe
+// stack.
+//
+// 3) Protection via information hiding on 64 bit architectures: the location
+// of the safe stack(s) can be randomized through secure mechanisms, and the
+// leakage of the stack pointer can be prevented. Currently, libc can leak the
+// stack pointer in several ways (e.g. in longjmp, signal handling, user-level
+// context switching related functions, etc.). These can be fixed in libc and
+// in other low-level libraries, by either eliminating the escaping/dumping of
+// the stack pointer (i.e., %rsp) when that's possible, or by using
+// encryption/PTR_MANGLE (XOR-ing the dumped stack pointer with another secret
+// we control and protect better, as is already done for setjmp in glibc.)
+// Furthermore, a static machine code level verifier can be ran after code
+// generation to make sure that the stack pointer is never written to memory,
+// or if it is, its written on the safe stack.
+//
+// Finally, while the Unsafe Stack pointer is currently stored in a thread
+// local variable, with libc support it could be stored in the TCB (thread
+// control block) as well, eliminating another level of indirection and making
+// such accesses faster. Alternatively, dedicating a separate register for
+// storing it would also be possible.
 
 /// Minimum stack alignment for the unsafe stack.
 const unsigned kStackAlign = 16;
 
 /// Default size of the unsafe stack. This value is only used if the stack
 /// size rlimit is set to infinity.
 const unsigned kDefaultUnsafeStackSize = 0x2800000;