[tip: x86/sgx] selftests/sgx: Improve error detection and messages

From: tip-bot2 for Dave Hansen
Date: Fri Mar 19 2021 - 14:46:31 EST

The following commit has been merged into the x86/sgx branch of tip:

Commit-ID: 4284f7acb78bfb0e0c26a2b78e2b2c3d68fccd6f
Gitweb: https://git.kernel.org/tip/4284f7acb78bfb0e0c26a2b78e2b2c3d68fccd6f
Author: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Thu, 18 Mar 2021 12:43:01 -07:00
Committer: Borislav Petkov <bp@xxxxxxx>
CommitterDate: Fri, 19 Mar 2021 19:23:41 +01:00

selftests/sgx: Improve error detection and messages

The SGX device file (/dev/sgx_enclave) is unusual in that it requires
execute permissions. It has to be both "chmod +x" *and* be on a
filesystem without 'noexec'.

In the future, udev and systemd should get updates to set up systems
automatically. But, for now, nobody's systems do this automatically,
and everybody gets error messages like this when running ./test_sgx:

0x0000000000000000 0x0000000000002000 0x03
0x0000000000002000 0x0000000000001000 0x05
0x0000000000003000 0x0000000000003000 0x03
mmap() failed, errno=1.

That isn't very user friendly, even for forgetful kernel developers.

Further, the test case is rather haphazard about its use of fprintf()
versus perror().

Improve the error messages. Use perror() where possible. Lastly,
do some sanity checks on opening and mmap()ing the device file so
that we can get a decent error message out to the user.

Now, if your user doesn't have permission, you'll get the following:

$ ls -l /dev/sgx_enclave
crw------- 1 root root 10, 126 Mar 18 11:29 /dev/sgx_enclave
$ ./test_sgx
Unable to open /dev/sgx_enclave: Permission denied

If you then 'chown dave:dave /dev/sgx_enclave' (or whatever), but
you leave execute permissions off, you'll get:

$ ls -l /dev/sgx_enclave
crw------- 1 dave dave 10, 126 Mar 18 11:29 /dev/sgx_enclave
$ ./test_sgx
no execute permissions on device file

If you fix that with "chmod ug+x /dev/sgx" but you leave /dev as
noexec, you'll get this:

$ mount | grep "/dev .*noexec"
udev on /dev type devtmpfs (rw,nosuid,noexec,...)
$ ./test_sgx
ERROR: mmap for exec: Operation not permitted
mmap() succeeded for PROT_READ, but failed for PROT_EXEC
check that user has execute permissions on /dev/sgx_enclave and
that /dev does not have noexec set: 'mount | grep "/dev .*noexec"'

That can be fixed with:

mount -o remount,noexec /devESC

Hopefully, the combination of better error messages and the search
engines indexing this message will help people fix their systems
until we do this properly.

[ bp: Improve error messages more. ]

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210318194301.11D9A984@xxxxxxxxxxxxxxxxxx
---
tools/testing/selftests/sgx/load.c | 69 +++++++++++++++++++++++------
tools/testing/selftests/sgx/main.c | 2 +-
2 files changed, 56 insertions(+), 15 deletions(-)

diff --git a/tools/testing/selftests/sgx/load.c b/tools/testing/selftests/sgx/load.c
index 9d43b75..f441ac3 100644
--- a/tools/testing/selftests/sgx/load.c
+++ b/tools/testing/selftests/sgx/load.c
@@ -45,19 +45,19 @@ static bool encl_map_bin(const char *path, struct encl *encl)

fd = open(path, O_RDONLY);
if (fd == -1) {
- perror("open()");
+ perror("enclave executable open()");
return false;
}

ret = stat(path, &sb);
if (ret) {
- perror("stat()");
+ perror("enclave executable stat()");
goto err;
}

bin = mmap(NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
if (bin == MAP_FAILED) {
- perror("mmap()");
+ perror("enclave executable mmap()");
goto err;
}

@@ -90,8 +90,7 @@ static bool encl_ioc_create(struct encl *encl)
ioc.src = (unsigned long)secs;
rc = ioctl(encl->fd, SGX_IOC_ENCLAVE_CREATE, &ioc);
if (rc) {
- fprintf(stderr, "SGX_IOC_ENCLAVE_CREATE failed: errno=%d\n",
- errno);
+ perror("SGX_IOC_ENCLAVE_CREATE failed");
munmap((void *)secs->base, encl->encl_size);
return false;
}
@@ -116,31 +115,72 @@ static bool encl_ioc_add_pages(struct encl *encl, struct encl_segment *seg)

rc = ioctl(encl->fd, SGX_IOC_ENCLAVE_ADD_PAGES, &ioc);
if (rc < 0) {
- fprintf(stderr, "SGX_IOC_ENCLAVE_ADD_PAGES failed: errno=%d.\n",
- errno);
+ perror("SGX_IOC_ENCLAVE_ADD_PAGES failed");
return false;
}

return true;
}

+
+
bool encl_load(const char *path, struct encl *encl)
{
+ const char device_path[] = "/dev/sgx_enclave";
Elf64_Phdr *phdr_tbl;
off_t src_offset;
Elf64_Ehdr *ehdr;
+ struct stat sb;
+ void *ptr;
int i, j;
int ret;
+ int fd = -1;

memset(encl, 0, sizeof(*encl));

- ret = open("/dev/sgx_enclave", O_RDWR);
- if (ret < 0) {
- fprintf(stderr, "Unable to open /dev/sgx_enclave\n");
+ fd = open(device_path, O_RDWR);
+ if (fd < 0) {
+ perror("Unable to open /dev/sgx_enclave");
+ goto err;
+ }
+
+ ret = stat(device_path, &sb);
+ if (ret) {
+ perror("device file stat()");
+ goto err;
+ }
+
+ /*
+ * This just checks if the /dev file has these permission
+ * bits set. It does not check that the current user is
+ * the owner or in the owning group.
+ */
+ if (!(sb.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))) {
+ fprintf(stderr, "no execute permissions on device file %s\n", device_path);
+ goto err;
+ }
+
+ ptr = mmap(NULL, PAGE_SIZE, PROT_READ, MAP_SHARED, fd, 0);
+ if (ptr == (void *)-1) {
+ perror("mmap for read");
+ goto err;
+ }
+ munmap(ptr, PAGE_SIZE);
+
+#define ERR_MSG \
+"mmap() succeeded for PROT_READ, but failed for PROT_EXEC.\n" \
+" Check that current user has execute permissions on %s and \n" \
+" that /dev does not have noexec set: mount | grep \"/dev .*noexec\"\n" \
+" If so, remount it executable: mount -o remount,exec /dev\n\n"
+
+ ptr = mmap(NULL, PAGE_SIZE, PROT_EXEC, MAP_SHARED, fd, 0);
+ if (ptr == (void *)-1) {
+ fprintf(stderr, ERR_MSG, device_path);
goto err;
}
+ munmap(ptr, PAGE_SIZE);

- encl->fd = ret;
+ encl->fd = fd;

if (!encl_map_bin(path, encl))
goto err;
@@ -217,6 +257,8 @@ bool encl_load(const char *path, struct encl *encl)
return true;

err:
+ if (fd != -1)
+ close(fd);
encl_delete(encl);
return false;
}
@@ -229,7 +271,7 @@ static bool encl_map_area(struct encl *encl)
area = mmap(NULL, encl_size * 2, PROT_NONE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (area == MAP_FAILED) {
- perror("mmap");
+ perror("reservation mmap()");
return false;
}

@@ -268,8 +310,7 @@ bool encl_build(struct encl *encl)
ioc.sigstruct = (uint64_t)&encl->sigstruct;
ret = ioctl(encl->fd, SGX_IOC_ENCLAVE_INIT, &ioc);
if (ret) {
- fprintf(stderr, "SGX_IOC_ENCLAVE_INIT failed: errno=%d\n",
- errno);
+ perror("SGX_IOC_ENCLAVE_INIT failed");
return false;
}

diff --git a/tools/testing/selftests/sgx/main.c b/tools/testing/selftests/sgx/main.c
index 724cec7..b117bb8 100644
--- a/tools/testing/selftests/sgx/main.c
+++ b/tools/testing/selftests/sgx/main.c
@@ -195,7 +195,7 @@ int main(int argc, char *argv[], char *envp[])
addr = mmap((void *)encl.encl_base + seg->offset, seg->size,
seg->prot, MAP_SHARED | MAP_FIXED, encl.fd, 0);
if (addr == MAP_FAILED) {
- fprintf(stderr, "mmap() failed, errno=%d.\n", errno);
+ perror("mmap() segment failed");
exit(KSFT_FAIL);
}
}