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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id 8sm11225429pgc.13.2019.06.17.21.55.06 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 17 Jun 2019 21:55:06 -0700 (PDT) From: Kees Cook To: Thomas Gleixner Cc: Kees Cook , Linus Torvalds , x86@kernel.org, Peter Zijlstra , Dave Hansen , linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com Subject: [PATCH v3 1/3] lkdtm: Check for SMEP clearing protections Date: Mon, 17 Jun 2019 21:55:01 -0700 Message-Id: <20190618045503.39105-2-keescook@chromium.org> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20190618045503.39105-1-keescook@chromium.org> References: <20190618045503.39105-1-keescook@chromium.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 : 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 [ 79.601810] ------------[ cut here ]------------ [ 79.603421] Attempt to unpin cr4 bits: 100000; bypass attack?! ... [ 79.650170] ---[ end trace 2452ca0f6126242e ]--- [ 79.652281] lkdtm: ok: SMEP did not get cleared [ 79.654344] lkdtm: trying to clear SMEP with call gadget [ 79.655937] lkdtm: ok: SMEP removal was reverted Signed-off-by: Kees Cook --- drivers/misc/lkdtm/bugs.c | 66 ++++++++++++++++++++++++++++++++++++++ drivers/misc/lkdtm/core.c | 1 + drivers/misc/lkdtm/lkdtm.h | 1 + 3 files changed, 68 insertions(+) diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c index 7eebbdfbcacd..3edf4464b9fc 100644 --- a/drivers/misc/lkdtm/bugs.c +++ b/drivers/misc/lkdtm/bugs.c @@ -255,3 +255,69 @@ 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 the middle of native_write_cr4() where the + * cr4 write happens, skipping the 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 b51cf182b031..58cfd713f8dc 100644 --- a/drivers/misc/lkdtm/core.c +++ b/drivers/misc/lkdtm/core.c @@ -123,6 +123,7 @@ static const struct crashtype crashtypes[] = { CRASHTYPE(CORRUPT_LIST_ADD), CRASHTYPE(CORRUPT_LIST_DEL), CRASHTYPE(CORRUPT_USER_DS), + CRASHTYPE(UNSET_SMEP), CRASHTYPE(CORRUPT_STACK), CRASHTYPE(CORRUPT_STACK_STRONG), CRASHTYPE(STACK_GUARD_PAGE_LEADING), diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h index b69ee004a3f7..d7eb5a8f1da4 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); -- 2.17.1