diff options
author | Kees Cook <keescook@chromium.org> | 2019-06-22 23:18:23 +0300 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2019-06-23 08:05:56 +0300 |
commit | 06b32fdb030989c45bb9dad685b794bf2395d53a (patch) | |
tree | c73030b5bb87d2923868488574fbd3bf2093046c | |
parent | 1909a671dbc3606685b1daf8b22a16f65ea7edda (diff) | |
download | linux-06b32fdb030989c45bb9dad685b794bf2395d53a.tar.xz |
lkdtm: Check for SMEP clearing protections
This adds an x86-specific test for pinned cr4 bits. A successful test
will validate pinning and check the ROP-style call-middle-of-function
defense, if needed. For example, in the case of native_write_cr4()
looking like this:
ffffffff8171bce0 <native_write_cr4>:
ffffffff8171bce0: 48 8b 35 79 46 f2 00 mov 0xf24679(%rip),%rsi
ffffffff8171bce7: 48 09 f7 or %rsi,%rdi
ffffffff8171bcea: 0f 22 e7 mov %rdi,%cr4
...
ffffffff8171bd5a: c3 retq
The UNSET_SMEP test will jump to ffffffff8171bcea (the mov to cr4)
instead of ffffffff8171bce0 (native_write_cr4() entry) to simulate a
direct-call bypass attempt.
Expected successful results:
# echo UNSET_SMEP > /sys/kernel/debug/provoke-crash/DIRECT
# dmesg
[ 79.594433] lkdtm: Performing direct entry UNSET_SMEP
[ 79.596459] lkdtm: trying to clear SMEP normally
[ 79.598406] lkdtm: ok: SMEP did not get cleared
[ 79.599981] lkdtm: trying to clear SMEP with call gadget
[ 79.601810] ------------[ cut here ]------------
[ 79.603421] Attempt to unpin cr4 bits: 100000; bypass attack?!
...
[ 79.650170] ---[ end trace 2452ca0f6126242e ]---
[ 79.650937] lkdtm: ok: SMEP removal was reverted
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r-- | drivers/misc/lkdtm/bugs.c | 66 | ||||
-rw-r--r-- | drivers/misc/lkdtm/core.c | 1 | ||||
-rw-r--r-- | drivers/misc/lkdtm/lkdtm.h | 1 |
3 files changed, 68 insertions, 0 deletions
diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c index 17f839dee976..3741b783fe44 100644 --- a/drivers/misc/lkdtm/bugs.c +++ b/drivers/misc/lkdtm/bugs.c @@ -266,3 +266,69 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void) pr_err("FAIL: accessed page after stack!\n"); } + +void lkdtm_UNSET_SMEP(void) +{ +#ifdef CONFIG_X86_64 +#define MOV_CR4_DEPTH 64 + void (*direct_write_cr4)(unsigned long val); + unsigned char *insn; + unsigned long cr4; + int i; + + cr4 = native_read_cr4(); + + if ((cr4 & X86_CR4_SMEP) != X86_CR4_SMEP) { + pr_err("FAIL: SMEP not in use\n"); + return; + } + cr4 &= ~(X86_CR4_SMEP); + + pr_info("trying to clear SMEP normally\n"); + native_write_cr4(cr4); + if (cr4 == native_read_cr4()) { + pr_err("FAIL: pinning SMEP failed!\n"); + cr4 |= X86_CR4_SMEP; + pr_info("restoring SMEP\n"); + native_write_cr4(cr4); + return; + } + pr_info("ok: SMEP did not get cleared\n"); + + /* + * To test the post-write pinning verification we need to call + * directly into the middle of native_write_cr4() where the + * cr4 write happens, skipping any pinning. This searches for + * the cr4 writing instruction. + */ + insn = (unsigned char *)native_write_cr4; + for (i = 0; i < MOV_CR4_DEPTH; i++) { + /* mov %rdi, %cr4 */ + if (insn[i] == 0x0f && insn[i+1] == 0x22 && insn[i+2] == 0xe7) + break; + /* mov %rdi,%rax; mov %rax, %cr4 */ + if (insn[i] == 0x48 && insn[i+1] == 0x89 && + insn[i+2] == 0xf8 && insn[i+3] == 0x0f && + insn[i+4] == 0x22 && insn[i+5] == 0xe0) + break; + } + if (i >= MOV_CR4_DEPTH) { + pr_info("ok: cannot locate cr4 writing call gadget\n"); + return; + } + direct_write_cr4 = (void *)(insn + i); + + pr_info("trying to clear SMEP with call gadget\n"); + direct_write_cr4(cr4); + if (native_read_cr4() & X86_CR4_SMEP) { + pr_info("ok: SMEP removal was reverted\n"); + } else { + pr_err("FAIL: cleared SMEP not detected!\n"); + cr4 |= X86_CR4_SMEP; + pr_info("restoring SMEP\n"); + native_write_cr4(cr4); + } +#else + pr_err("FAIL: this test is x86_64-only\n"); +#endif +} diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c index 8a1428d4f138..a5a04185a3d8 100644 --- a/drivers/misc/lkdtm/core.c +++ b/drivers/misc/lkdtm/core.c @@ -114,6 +114,7 @@ static const struct crashtype crashtypes[] = { CRASHTYPE(CORRUPT_USER_DS), CRASHTYPE(STACK_GUARD_PAGE_LEADING), CRASHTYPE(STACK_GUARD_PAGE_TRAILING), + CRASHTYPE(UNSET_SMEP), CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE), CRASHTYPE(OVERWRITE_ALLOCATION), CRASHTYPE(WRITE_AFTER_FREE), diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h index 23dc565b4307..bbcd370786d4 100644 --- a/drivers/misc/lkdtm/lkdtm.h +++ b/drivers/misc/lkdtm/lkdtm.h @@ -26,6 +26,7 @@ void lkdtm_CORRUPT_LIST_DEL(void); void lkdtm_CORRUPT_USER_DS(void); void lkdtm_STACK_GUARD_PAGE_LEADING(void); void lkdtm_STACK_GUARD_PAGE_TRAILING(void); +void lkdtm_UNSET_SMEP(void); /* lkdtm_heap.c */ void lkdtm_OVERWRITE_ALLOCATION(void); |