Commit-ID: 76fc5e7b2355af167dea1a32e93c57fc37900a5b
Gitweb: http://git.kernel.org/tip/76fc5e7b2355af167dea1a32e93c57fc37900a5b
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Fri, 4 Sep 2015 17:00:43 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 5 Sep 2015 09:01:16 +0200
x86/vm86: Block non-root vm86(old) if mmap_min_addr != 0
vm86 exposes an interesting attack surface against the entry
code. Since vm86 is mostly useless anyway if mmap_min_addr != 0,
just turn it off in that case.
There are some reports that vbetool can work despite setting
mmap_min_addr to zero. This shouldn't break that use case,
as CAP_SYS_RAWIO already overrides mmap_min_addr.
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Josh Boyer <jwboyer@xxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stas Sergeev <stsp@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/vm86_32.c | 27 +++++++++++++++++++++++++++
tools/testing/selftests/x86/entry_from_vm86.c | 5 +++--
2 files changed, 30 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index abd8b856..5246193 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -45,6 +45,7 @@
#include <linux/audit.h>
#include <linux/stddef.h>
#include <linux/slab.h>
+#include <linux/security.h>
#include <asm/uaccess.h>
#include <asm/io.h>
@@ -232,6 +233,32 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
struct pt_regs *regs = current_pt_regs();
unsigned long err = 0;
+ err = security_mmap_addr(0);
+ if (err) {
+ /*
+ * vm86 cannot virtualize the address space, so vm86 users
+ * need to manage the low 1MB themselves using mmap. Given
+ * that BIOS places important data in the first page, vm86
+ * is essentially useless if mmap_min_addr != 0. DOSEMU,
+ * for example, won't even bother trying to use vm86 if it
+ * can't map a page at virtual address 0.
+ *
+ * To reduce the available kernel attack surface, simply
+ * disallow vm86(old) for users who cannot mmap at va 0.
+ *
+ * The implementation of security_mmap_addr will allow
+ * suitably privileged users to map va 0 even if
+ * vm.mmap_min_addr is set above 0, and we want this
+ * behavior for vm86 as well, as it ensures that legacy
+ * tools like vbetool will not fail just because of
+ * vm.mmap_min_addr.
+ */
+ pr_info_once("Denied a call to vm86(old) from %s[%d] (uid: %d). Set the vm.mmap_min_addr sysctl to 0 and/or adjust LSM mmap_min_addr policy to enable vm86 if you are using a vm86-based DOS emulator.\n",
+ current->comm, task_pid_nr(current),
+ from_kuid_munged(&init_user_ns, current_uid()));
+ return -EPERM;
+ }
+
if (!vm86) {
if (!(vm86 = kzalloc(sizeof(*vm86), GFP_KERNEL)))
return -ENOMEM;
diff --git a/tools/testing/selftests/x86/entry_from_vm86.c b/tools/testing/selftests/x86/entry_from_vm86.c
index 9a43a59..421c607 100644
--- a/tools/testing/selftests/x86/entry_from_vm86.c
+++ b/tools/testing/selftests/x86/entry_from_vm86.c
@@ -116,8 +116,9 @@ static bool do_test(struct vm86plus_struct *v86, unsigned long eip,
v86->regs.eip = eip;
ret = vm86(VM86_ENTER, v86);
- if (ret == -1 && errno == ENOSYS) {
- printf("[SKIP]\tvm86 not supported\n");
+ if (ret == -1 && (errno == ENOSYS || errno == EPERM)) {
+ printf("[SKIP]\tvm86 %s\n",
+ errno == ENOSYS ? "not supported" : "not allowed");
return false;
}
--
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