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Fix seccomp sandbox rules for openat #27315

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The need for casting negative syscall arguments depends on the
glibc version. This affects the rules for the openat syscall which
uses the constant AT_FDCWD that is defined as a negative number.
This commit adds logic to only apply the cast when necessary, on
glibc versions from 2.27 onwards.
This commit is contained in:
Daniel Pinto 2020-07-01 20:30:04 +01:00
parent c9751e2611
commit d75e7daaab
2 changed files with 38 additions and 10 deletions
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6
changes/bug27315 Normal file
View File

@ -0,0 +1,6 @@
o Minor bugfixes (linux seccomp2 sandbox):
- Fix a regression on sandboxing rules for the openat() syscall.
The fix for bug 25440 fixed the problem on systems with glibc >=
2.27 but broke tor on previous versions of glibc. We now apply
the correct seccomp rule according to the running glibc version.
Patch from Daniel Pinto. Fixes bug 27315; bugfix on 0.3.5.11.

42
src/lib/sandbox/sandbox.c
View File

@ -134,6 +134,10 @@ 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. */
#define SCMP_CMP_NEG(a,op,b) (libc_negative_constant_needs_cast() ? \
(SCMP_CMP((a), (op), (unsigned int)(b))) : \
(SCMP_CMP_STR((a), (op), (b))))
/** Variable used for storing all syscall numbers that will be allowed with the
* stage 1 general Tor sandbox.
@ -424,31 +428,49 @@ sb_mmap2(scmp_filter_ctx ctx, sandbox_cfg_t *filter)
#endif
#endif
/* Return true if we think we're running with a libc that always uses
* openat on linux. */
/* Return true the libc version is greater or equal than
* <b>major</b>.<b>minor</b>. Returns false otherwise. */
static int
libc_uses_openat_for_everything(void)
is_libc_at_least(int major, int minor)
{
#ifdef CHECK_LIBC_VERSION
const char *version = gnu_get_libc_version();
if (version == NULL)
return 0;
int major = -1;
int minor = -1;
int libc_major = -1;
int libc_minor = -1;
tor_sscanf(version, "%d.%d", &major, &minor);
if (major >= 3)
tor_sscanf(version, "%d.%d", &libc_major, &libc_minor);
if (libc_major > major)
return 1;
else if (major == 2 && minor >= 26)
else if (libc_major == major && libc_minor >= minor)
return 1;
else
return 0;
#else /* !(defined(CHECK_LIBC_VERSION)) */
(void)major;
(void)minor;
return 0;
#endif /* defined(CHECK_LIBC_VERSION) */
}
/* Return true if we think we're running with a libc that always uses
* openat on linux. */
static int
libc_uses_openat_for_everything(void)
{
return is_libc_at_least(2, 26);
}
/* 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)
{
return is_libc_at_least(2, 27);
}
/** 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
@ -456,7 +478,7 @@ 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(0, SCMP_CMP_EQ, (unsigned int)AT_FDCWD),
SCMP_CMP_NEG(0, SCMP_CMP_EQ, AT_FDCWD),
SCMP_CMP_STR(1, SCMP_CMP_EQ, file));
} else {
return seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open),
@ -592,7 +614,7 @@ 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(0, SCMP_CMP_EQ, AT_FDCWD),
SCMP_CMP_NEG(0, SCMP_CMP_EQ, 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));