[Qemu-devel] [PATCH] Add a few argument filters to the seccomp sandbox

[Qemu-devel] [PATCH] Add a few argument filters to the seccomp sandbox

[namnamc]

This patch here adds argument filtering for three syscalls:
madvise(), shmget(), and shmctl().

The madvise() flags may need a few additions, but I couldn't find any common
cases where the extra flags were used. The only additions were ones I found by
grepping through the source code for all madvise-related flags:
$ grep -hro MADV_[A-Z]* qemu-2.3.0 | sort -u
MADV_DODUMP
MADV_DONTDUMP
MADV_DONTFORK
MADV_DONTNEED
MADV_HUGEPAGE
MADV_INVALID
MADV_MERGEABLE
MADV_UNMERGEABLE
MADV_WILLNEED

The rest of the flags were found by stracing Qemu during some common uses
cases. The tests were done on an x86_64 hardened Gentoo machine for the host,
and the guests were a variety of Debian-based Linux distros. For the purpose
of these syscalls, I don't think the guest OS matters much.

Signed-off-by: Namsun Ch'o <namnamc@safe-mail.net>
---
This is my first patch to Qemu. I have many more syscalls filtered, but was
told on IRC to send in patches broken into smaller chunks with just a few new
filters each time to make review easier. This is the first one. The next one
will be for ioctl() which is possibly the most important considering its rich
history of abuse.

diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index f9de0d3..0c11580 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -14,6 +14,8 @@
  */
 #include <stdio.h>
 #include <seccomp.h>
+#include <linux/ipc.h>
+#include <asm-generic/mman-common.h>
 #include "sysemu/seccomp.h"
 
 struct QemuSeccompSyscall {
@@ -105,7 +107,6 @@ static const struct QemuSeccompSyscall seccomp_whitelist[] = {
     { SCMP_SYS(rt_sigreturn), 245 },
     { SCMP_SYS(sync), 245 },
     { SCMP_SYS(pread64), 245 },
-    { SCMP_SYS(madvise), 245 },
     { SCMP_SYS(set_robust_list), 245 },
     { SCMP_SYS(lseek), 245 },
     { SCMP_SYS(pselect6), 245 },
@@ -224,11 +225,9 @@ static const struct QemuSeccompSyscall seccomp_whitelist[] = {
     { SCMP_SYS(arch_prctl), 240 },
     { SCMP_SYS(mkdir), 240 },
     { SCMP_SYS(fchmod), 240 },
-    { SCMP_SYS(shmget), 240 },
     { SCMP_SYS(shmat), 240 },
     { SCMP_SYS(shmdt), 240 },
     { SCMP_SYS(timerfd_create), 240 },
-    { SCMP_SYS(shmctl), 240 },
     { SCMP_SYS(mlockall), 240 },
     { SCMP_SYS(mlock), 240 },
     { SCMP_SYS(munlock), 240 },
@@ -264,6 +263,49 @@ int seccomp_start(void)
         }
     }
 
+    /* madvise */
+    static const int madvise_flags[] = {
+        MADV_DONTFORK,
+        MADV_DONTNEED,
+        MADV_HUGEPAGE,
+        MADV_MERGEABLE,
+    };
+    for (i = 0; i < ARRAY_SIZE(madvise_flags); i++) {
+        rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(madvise), 1,
+            SCMP_A2(SCMP_CMP_EQ, madvise_flags[i]));
+        if (rc < 0) {
+            goto seccomp_return;
+        }
+    }
+    rc = seccomp_syscall_priority(ctx, SCMP_SYS(madvise), 245);
+    if (rc < 0) {
+        goto seccomp_return;
+    }
+
+    /* shmget */
+    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmget), 2,
+        SCMP_A0(SCMP_CMP_EQ, IPC_PRIVATE),
+        SCMP_A2(SCMP_CMP_EQ, IP_CREAT|0777));
+    if (rc < 0) {
+        goto seccomp_return;
+    }
+    rc = seccomp_syscall_priority(ctx, SCMP_SYS(shmget), 240);
+    if (rc < 0) {
+        goto seccomp_return;
+    }
+
+    /* shmctl */
+    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmctl), 2,
+        SCMP_A1(SCMP_CMP_EQ, IPC_RMID),
+        SCMP_A2(SCMP_CMP_EQ, 0));
+    if (rc < 0) {
+        goto seccomp_return;
+    }
+    rc = seccomp_syscall_priority(ctx, SCMP_SYS(shmctl), 240);
+    if (rc < 0) {
+        goto seccomp_return;
+    }
+
     rc = seccomp_load(ctx);
 
   seccomp_return:

Re: [Qemu-devel] [PATCH] Add a few argument filters to the seccomp sandbox

[Daniel P. Berrange]

(Adding Paul Moore to CC since he's done a lot of work on syscall
 whitelisting in QEMU)

On Wed, Sep 09, 2015 at 09:55:33PM -0400, namnamc@Safe-mail.net wrote:
> This patch here adds argument filtering for three syscalls:
> madvise(), shmget(), and shmctl().
> 
> The madvise() flags may need a few additions, but I couldn't find any common
> cases where the extra flags were used. The only additions were ones I found by
> grepping through the source code for all madvise-related flags:

The current intention of the seccomp filter in QEMU, is that /all/ existing
QEMU features continue to work unchanged. So even if a flag is used in a
seemingly uncommon code path, we still need to allow that in a seccomp
filter.

Soo all these are needed:

> $ grep -hro MADV_[A-Z]* qemu-2.3.0 | sort -u
> MADV_DODUMP
> MADV_DONTDUMP
> MADV_DONTFORK
> MADV_DONTNEED
> MADV_HUGEPAGE
> MADV_INVALID

This one isn't a real constant as the syscall level

> MADV_MERGEABLE
> MADV_UNMERGEABLE
> MADV_WILLNEED


One thing to be wary off when checking for used syscalls in the
source, is that a large amount of QEMU functionality is provided
via 3rd party libraries, especially the crypto/TLS code, SPICE
server, and various storage drivers for RBD, gluster, iSCSI
and GTK/SDL for the UI.

Probably not an issue for madvise, but this is in fact where
the usage of shmget comes from - QEMU never directly uses it
but (at least) GTK does call it. So in this case, we need to
check at least the GTK source to ensure we covered all args.
I wouldn't be surprised if SDL uses it too. This gets kind of
tedious quite quickly....


> diff --git a/qemu-seccomp.c b/qemu-seccomp.c
> index f9de0d3..0c11580 100644
> --- a/qemu-seccomp.c
> +++ b/qemu-seccomp.c
> @@ -264,6 +263,49 @@ int seccomp_start(void)
>          }
>      }
>  
> +    /* madvise */
> +    static const int madvise_flags[] = {
> +        MADV_DONTFORK,
> +        MADV_DONTNEED,
> +        MADV_HUGEPAGE,
> +        MADV_MERGEABLE,

So we need to add DODUMP, DONTDUMP, UNMERGABLE and WILLNEED here. That
is still stricter than the previous allow-everything rule, so a net
win.

> +    };
> +    for (i = 0; i < ARRAY_SIZE(madvise_flags); i++) {
> +        rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(madvise), 1,
> +            SCMP_A2(SCMP_CMP_EQ, madvise_flags[i]));
> +        if (rc < 0) {
> +            goto seccomp_return;
> +        }
> +    }
> +    rc = seccomp_syscall_priority(ctx, SCMP_SYS(madvise), 245);
> +    if (rc < 0) {
> +        goto seccomp_return;
> +    }


> +
> +    /* shmget */
> +    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmget), 2,
> +        SCMP_A0(SCMP_CMP_EQ, IPC_PRIVATE),
> +        SCMP_A2(SCMP_CMP_EQ, IP_CREAT|0777));

I'm not familiar with semantics of these seccomp rules, but is this
saying that the second arg must be exactly equal to IP_CREAT|0777 ?
If the app passes IP_CREAT|0600, would that be permitted instead ?
The latter is what I see gtk2 source code passing for mode.

> +    if (rc < 0) {
> +        goto seccomp_return;
> +    }
> +    rc = seccomp_syscall_priority(ctx, SCMP_SYS(shmget), 240);
> +    if (rc < 0) {
> +        goto seccomp_return;
> +    }
> +
> +    /* shmctl */
> +    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmctl), 2,
> +        SCMP_A1(SCMP_CMP_EQ, IPC_RMID),
> +        SCMP_A2(SCMP_CMP_EQ, 0));
> +    if (rc < 0) {
> +        goto seccomp_return;
> +    }
> +    rc = seccomp_syscall_priority(ctx, SCMP_SYS(shmctl), 240);
> +    if (rc < 0) {
> +        goto seccomp_return;
> +    }
> +


Regards,
Daniel
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Re: [Qemu-devel] [PATCH] Add a few argument filters to the seccomp sandbox

[Paul Moore]

On Thursday, September 10, 2015 03:48:52 PM Daniel P. Berrange wrote:
> On Wed, Sep 09, 2015 at 09:55:33PM -0400, namnamc@Safe-mail.net wrote:
> > +
> > +    /* shmget */
> > +    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmget), 2,
> > +        SCMP_A0(SCMP_CMP_EQ, IPC_PRIVATE),
> > +        SCMP_A2(SCMP_CMP_EQ, IP_CREAT|0777));
> 
> I'm not familiar with semantics of these seccomp rules, but is this
> saying that the second arg must be exactly equal to IP_CREAT|0777 ?

Yes.

> If the app passes IP_CREAT|0600, would that be permitted instead ?
> The latter is what I see gtk2 source code passing for mode.

It wouldn't match the rule as written above, if it doesn't match any other 
configured rules it would hit the default filter action.

-- 
paul moore
security @ redhat

Re: [Qemu-devel] [PATCH] Add a few argument filters to the seccomp sandbox

[Namsun Ch'o]

> On Thursday, September 10, 2015 03:48:52 PM Daniel P. Berrange wrote:
> > On Wed, Sep 09, 2015 at 09:55:33PM -0400, namnamc@Safe-mail.net wrote:
> > > +
> > > +    /* shmget */
> > > +    rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(shmget), 2,
> > > +        SCMP_A0(SCMP_CMP_EQ, IPC_PRIVATE),
> > > +        SCMP_A2(SCMP_CMP_EQ, IP_CREAT|0777));
> > 
> > I'm not familiar with semantics of these seccomp rules, but is this
> > saying that the second arg must be exactly equal to IP_CREAT|0777 ?
> 
> Yes.
> 
> > If the app passes IP_CREAT|0600, would that be permitted instead ?
> > The latter is what I see gtk2 source code passing for mode.
> 
> It wouldn't match the rule as written above, if it doesn't match any other 
> configured rules it would hit the default filter action.
> 
> -- 
> paul moore
> security @ redhat

I'd already replied to that but in a new top-level thread because I had an
updated patch. Did I misunderstand the "contributing a patch" FAQ? Anyway,
I added the IP_CREAT|0600 rule to the v2 patch:

https://lists.nongnu.org/archive/html/qemu-devel/2015-09/msg02953.html