Re: [PATCH 7/9] Simplify bound checks in capabilities for copy_from_user

[Arjan van de Ven]

On Tue, 29 Sep 2009 15:55:49 +1000
Dave Airlie <airlied@xxxxxxxxx> wrote:

> On Sun, Sep 27, 2009 at 4:53 AM, Arjan van de Ven
> <arjan@xxxxxxxxxxxxx> wrote:
> > From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> > Subject: [PATCH 7/9] Simplify bound checks in capabilities for
> > copy_from_user CC: James Morris <jmorris@xxxxxxxxx>
> >
> > The capabilities syscall has a copy_from_user() call where gcc
> > currently cannot prove to itself that the copy is always within
> > bounds.
> >
> > This patch adds a very explicity bound check to prove to gcc that
> > this copy_from_user cannot overflow its destination buffer.
> >
> > Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> >
> > diff --git a/kernel/capability.c b/kernel/capability.c
> > index 4e17041..204f11f 100644
> > --- a/kernel/capability.c
> > +++ b/kernel/capability.c
> > @@ -238,7 +241,7 @@ SYSCALL_DEFINE2(capget, cap_user_header_t,
> > header, cap_user_data_t, dataptr) SYSCALL_DEFINE2(capset,
> > cap_user_header_t, header, const cap_user_data_t, data) {
> > Â Â Â Âstruct __user_cap_data_struct kdata[_KERNEL_CAPABILITY_U32S];
> > - Â Â Â unsigned i, tocopy;
> > + Â Â Â unsigned i, tocopy, copybytes;
> > Â Â Â Âkernel_cap_t inheritable, permitted, effective;
> > Â Â Â Âstruct cred *new;
> > Â Â Â Âint ret;
> > @@ -255,8 +258,11 @@ SYSCALL_DEFINE2(capset, cap_user_header_t,
> > header, const cap_user_data_t, data) if (pid != 0 && pid !=
> > task_pid_vnr(current)) return -EPERM;
> >
> > - Â Â Â if (copy_from_user(&kdata, data,
> > - Â Â Â Â Â Â Â Â Â Â Â Â Âtocopy * sizeof(struct
> > __user_cap_data_struct)))
> > + Â Â Â copybytes = tocopy * sizeof(struct __user_cap_data_struct);
> > + Â Â Â if (copybytes > _KERNEL_CAPABILITY_U32S)
> > + Â Â Â Â Â Â Â return -EFAULT;
> 
> This is broken, it breaks dbus at least for me. you compare bytes
> to u32s wrongly.
> 
> Dave.

good point

From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Subject: [PATCH 7/9] Simplify bound checks in capabilities for copy_from_user
CC: James Morris <jmorris@xxxxxxxxx>

The capabilities syscall has a copy_from_user() call where gcc currently
cannot prove to itself that the copy is always within bounds.

This patch adds a very explicity bound check to prove to gcc that 
this copy_from_user cannot overflow its destination buffer.

Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>

diff --git a/kernel/capability.c b/kernel/capability.c
index 4e17041..204f11f 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -238,7 +241,7 @@ SYSCALL_DEFINE2(capget, cap_user_header_t, header, cap_user_data_t, dataptr)
 SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
 {
 	struct __user_cap_data_struct kdata[_KERNEL_CAPABILITY_U32S];
-	unsigned i, tocopy;
+	unsigned i, tocopy, copybytes;
 	kernel_cap_t inheritable, permitted, effective;
 	struct cred *new;
 	int ret;
@@ -255,8 +258,11 @@ SYSCALL_DEFINE2(capset, cap_user_header_t, header, const cap_user_data_t, data)
 	if (pid != 0 && pid != task_pid_vnr(current))
 		return -EPERM;
 
-	if (copy_from_user(&kdata, data,
-			   tocopy * sizeof(struct __user_cap_data_struct)))
+	copybytes = tocopy * sizeof(struct __user_cap_data_struct);
+	if (copybytes > sizeof(kdata))
+		return -EFAULT;
+
+	if (copy_from_user(&kdata, data, copybytes))
 		return -EFAULT;
 
 	for (i = 0; i < tocopy; i++) {



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Arjan van de Ven 	Intel Open Source Technology Centre
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