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@linux-foundation.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index abd8b856bd2b50af307caa2eaad953dc80c4ed00..5246193519614dbd8d3a602544e8117376df05f7 100644 (file)
--- 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 9a43a59a9bb46b2944b20b26f1af6e8e405b5764..421c607a8856887d0f9f6048c2ab26bd9367efdd 100644 (file)
--- 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;
        }