sandbox: replace SCMP_CMP_NEG with masked equality checks

For some syscalls the kernel ABI uses 32 bit signed integers. Whether
these 32 bit integer values are sign extended or zero extended to the
native 64 bit register sizes is undefined and dependent on the {arch,
compiler, libc} being used. Instead of trying to detect which cases
zero-extend and which cases sign-extend, this commit uses a masked
equality check on the lower 32 bits of the value.

src/lib/sandbox/sandbox.c

@@ -141,10 +141,12 @@ static sandbox_cfg_t *filter_dynamic = NULL;
  * the high bits of the value might get masked out improperly. */
 #define SCMP_CMP_MASKED(a,b,c) \
   SCMP_CMP4((a), SCMP_CMP_MASKED_EQ, ~(scmp_datum_t)(b), (c))
-/* For negative constants, the rule to add depends on the glibc version. */
+/* Negative constants aren't consistently sign extended or zero extended.
-#define SCMP_CMP_NEG(a,op,b) (libc_negative_constant_needs_cast() ? \
+ * Different compilers, libc, and architectures behave differently. For cases
-                              (SCMP_CMP((a), (op), (unsigned int)(b))) : \
+ * where the kernel ABI uses a 32 bit integer, this macro can be used to
-                              (SCMP_CMP_STR((a), (op), (b))))
+ * mask-compare only the lower 32 bits of the value. */
+#define SCMP_CMP_LOWER32_EQ(a,b) \
+  SCMP_CMP4((a), SCMP_CMP_MASKED_EQ, 0xFFFFFFFF, (unsigned int)(b))
 
 /** Variable used for storing all syscall numbers that will be allowed with the
  * stage 1 general Tor sandbox.
@@ -513,19 +515,6 @@ libc_uses_openat_for_opendir(void)
          (is_libc_at_least(2, 15) && !is_libc_at_least(2, 22));
 }
 
-/* Return true if we think we're running with a libc that needs to cast
- * negative arguments like AT_FDCWD for seccomp rules. */
-static int
-libc_negative_constant_needs_cast(void)
-{
-#if defined(__aarch64__) && defined(__LP64__)
-  /* Existing glibc versions always sign-extend to 64 bits on AArch64. */
-  return 0;
-#else
-  return is_libc_at_least(2, 27);
-#endif
-}
-
 /** Allow a single file to be opened.  If <b>use_openat</b> is true,
  * we're using a libc that remaps all the opens into openats. */
 static int
@@ -533,7 +522,7 @@ allow_file_open(scmp_filter_ctx ctx, int use_openat, const char *file)
 {
   if (use_openat) {
     return seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat),
-                              SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+                              SCMP_CMP_LOWER32_EQ(0, AT_FDCWD),
                               SCMP_CMP_STR(1, SCMP_CMP_EQ, file));
   } else {
     return seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open),
@@ -627,7 +616,7 @@ sb_fchmodat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
     if (param != NULL && param->prot == 1 && param->syscall
         == SCMP_SYS(fchmodat)) {
       rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchmodat),
-          SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+          SCMP_CMP_LOWER32_EQ(0, AT_FDCWD),
           SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value));
       if (rc != 0) {
         log_err(LD_BUG,"(Sandbox) failed to add fchmodat syscall, received "
@@ -705,7 +694,7 @@ sb_fchownat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
     if (param != NULL && param->prot == 1 && param->syscall
         == SCMP_SYS(fchownat)) {
       rc = seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(fchownat),
-          SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+          SCMP_CMP_LOWER32_EQ(0, AT_FDCWD),
           SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value));
       if (rc != 0) {
         log_err(LD_BUG,"(Sandbox) failed to add fchownat syscall, received "
@@ -767,9 +756,9 @@ sb_renameat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
         param->syscall == SCMP_SYS(renameat)) {
 
       rc = seccomp_rule_add_4(ctx, SCMP_ACT_ALLOW, SCMP_SYS(renameat),
-            SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+            SCMP_CMP_LOWER32_EQ(0, AT_FDCWD),
             SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value),
-            SCMP_CMP_NEG(2, SCMP_CMP_EQ, AT_FDCWD),
+            SCMP_CMP_LOWER32_EQ(2, AT_FDCWD),
             SCMP_CMP_STR(3, SCMP_CMP_EQ, param->value2));
       if (rc != 0) {
         log_err(LD_BUG,"(Sandbox) failed to add renameat syscall, received "
@@ -799,7 +788,7 @@ sb_openat(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
     if (param != NULL && param->prot == 1 && param->syscall
         == SCMP_SYS(openat)) {
       rc = seccomp_rule_add_3(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat),
-          SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
+          SCMP_CMP_LOWER32_EQ(0, AT_FDCWD),
           SCMP_CMP_STR(1, SCMP_CMP_EQ, param->value),
           SCMP_CMP(2, SCMP_CMP_EQ, O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|
               O_CLOEXEC));