* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 21:55 ` [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges Eric Biggers
@ 2018-11-14 22:04 ` Jann Horn
2018-11-14 22:28 ` Dmitry Torokhov
2018-11-15 12:09 ` David Herrmann
2018-11-19 12:52 ` Jiri Kosina
2 siblings, 1 reply; 25+ messages in thread
From: Jann Horn @ 2018-11-14 22:04 UTC (permalink / raw)
To: ebiggers
Cc: dh.herrmann, Jiri Kosina, benjamin.tissoires, linux-input,
kernel list, syzkaller-bugs, Dmitry Vyukov, dtor,
syzbot+72473edc9bf4eb1c6556, stable, Andy Lutomirski
On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <stable@vger.kernel.org> # v3.6+
> Cc: Jann Horn <jannh@google.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jann Horn <jannh@google.com>
> ---
> drivers/hid/uhid.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..051639c09f728 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -12,6 +12,7 @@
>
> #include <linux/atomic.h>
> #include <linux/compat.h>
> +#include <linux/cred.h>
> #include <linux/device.h>
> #include <linux/fs.h>
> #include <linux/hid.h>
> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>
> switch (uhid->input_buf.type) {
> case UHID_CREATE:
> + /*
> + * 'struct uhid_create_req' contains a __user pointer which is
> + * copied from, so it's unsafe to allow this with elevated
> + * privileges (e.g. from a setuid binary) or via kernel_write().
> + */
> + if (file->f_cred != current_cred() || uaccess_kernel()) {
> + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> + task_tgid_vnr(current), current->comm);
> + ret = -EACCES;
> + goto unlock;
> + }
> ret = uhid_dev_create(uhid, &uhid->input_buf);
> break;
> case UHID_CREATE2:
> --
> 2.19.1.930.g4563a0d9d0-goog
>
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 22:04 ` Jann Horn
@ 2018-11-14 22:28 ` Dmitry Torokhov
2018-11-14 22:37 ` Jann Horn
2018-11-14 23:00 ` Eric Biggers
0 siblings, 2 replies; 25+ messages in thread
From: Dmitry Torokhov @ 2018-11-14 22:28 UTC (permalink / raw)
To: jannh
Cc: ebiggers, dh.herrmann, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, lkml, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, luto
On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > From: Eric Biggers <ebiggers@google.com>
> >
> > When a UHID_CREATE command is written to the uhid char device, a
> > copy_from_user() is done from a user pointer embedded in the command.
> > When the address limit is KERNEL_DS, e.g. as is the case during
> > sys_sendfile(), this can read from kernel memory. Alternatively,
> > information can be leaked from a setuid binary that is tricked to write
> > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> >
> > No other commands in uhid_char_write() are affected by this bug and
> > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > UHID_CREATE only rather than to uhid_char_write() entirely.
> >
> > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > helpers fault on kernel addresses"), allowing this bug to be found.
> >
> > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <stable@vger.kernel.org> # v3.6+
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
>
> Reviewed-by: Jann Horn <jannh@google.com>
>
> > ---
> > drivers/hid/uhid.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > index 3c55073136064..051639c09f728 100644
> > --- a/drivers/hid/uhid.c
> > +++ b/drivers/hid/uhid.c
> > @@ -12,6 +12,7 @@
> >
> > #include <linux/atomic.h>
> > #include <linux/compat.h>
> > +#include <linux/cred.h>
> > #include <linux/device.h>
> > #include <linux/fs.h>
> > #include <linux/hid.h>
> > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> >
> > switch (uhid->input_buf.type) {
> > case UHID_CREATE:
> > + /*
> > + * 'struct uhid_create_req' contains a __user pointer which is
> > + * copied from, so it's unsafe to allow this with elevated
> > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > + */
uhid is a privileged interface so we would go from root to less
privileged (if at all). If non-privileged process can open uhid it can
construct virtual keyboard and inject whatever keystrokes it wants.
Also, instead of disallowing access, can we ensure that we switch back
to USER_DS before trying to load data from the user pointer?
> > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > + task_tgid_vnr(current), current->comm);
> > + ret = -EACCES;
> > + goto unlock;
> > + }
> > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > break;
> > case UHID_CREATE2:
> > --
> > 2.19.1.930.g4563a0d9d0-goog
> >
Thanks.
--
Dmitry
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 22:28 ` Dmitry Torokhov
@ 2018-11-14 22:37 ` Jann Horn
2018-11-14 22:46 ` Dmitry Torokhov
2018-11-14 23:00 ` Eric Biggers
1 sibling, 1 reply; 25+ messages in thread
From: Jann Horn @ 2018-11-14 22:37 UTC (permalink / raw)
To: dtor
Cc: ebiggers, dh.herrmann, Jiri Kosina, benjamin.tissoires,
linux-input, kernel list, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, Andy Lutomirski
On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <dtor@google.com> wrote:
> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > When a UHID_CREATE command is written to the uhid char device, a
> > > copy_from_user() is done from a user pointer embedded in the command.
> > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > information can be leaked from a setuid binary that is tricked to write
> > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > >
> > > No other commands in uhid_char_write() are affected by this bug and
> > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > UHID_CREATE only rather than to uhid_char_write() entirely.
[...]
> > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
[...]
> > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > >
> > > switch (uhid->input_buf.type) {
> > > case UHID_CREATE:
> > > + /*
> > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > + * copied from, so it's unsafe to allow this with elevated
> > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > + */
>
> uhid is a privileged interface so we would go from root to less
> privileged (if at all). If non-privileged process can open uhid it can
> construct virtual keyboard and inject whatever keystrokes it wants.
>
> Also, instead of disallowing access, can we ensure that we switch back
> to USER_DS before trying to load data from the user pointer?
Does that even make sense? You are using some deprecated legacy
interface; you interact with it by splicing a request from something
like a file or a pipe into the uhid device; but the request you're
splicing through contains a pointer into userspace memory? Do you know
of anyone who is actually doing that? If not, anyone who does want to
do this for some reason in the future can just go use UHID_CREATE2
instead.
> > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > + task_tgid_vnr(current), current->comm);
> > > + ret = -EACCES;
> > > + goto unlock;
> > > + }
> > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > break;
> > > case UHID_CREATE2:
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 22:37 ` Jann Horn
@ 2018-11-14 22:46 ` Dmitry Torokhov
2018-11-15 0:39 ` Andy Lutomirski
0 siblings, 1 reply; 25+ messages in thread
From: Dmitry Torokhov @ 2018-11-14 22:46 UTC (permalink / raw)
To: jannh
Cc: ebiggers, dh.herrmann, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, lkml, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, luto
On Wed, Nov 14, 2018 at 2:38 PM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <dtor@google.com> wrote:
> > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > information can be leaked from a setuid binary that is tricked to write
> > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > >
> > > > No other commands in uhid_char_write() are affected by this bug and
> > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> [...]
> > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> [...]
> > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > >
> > > > switch (uhid->input_buf.type) {
> > > > case UHID_CREATE:
> > > > + /*
> > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > + * copied from, so it's unsafe to allow this with elevated
> > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > + */
> >
> > uhid is a privileged interface so we would go from root to less
> > privileged (if at all). If non-privileged process can open uhid it can
> > construct virtual keyboard and inject whatever keystrokes it wants.
> >
> > Also, instead of disallowing access, can we ensure that we switch back
> > to USER_DS before trying to load data from the user pointer?
>
> Does that even make sense? You are using some deprecated legacy
> interface; you interact with it by splicing a request from something
> like a file or a pipe into the uhid device; but the request you're
> splicing through contains a pointer into userspace memory? Do you know
> of anyone who is actually doing that? If not, anyone who does want to
> do this for some reason in the future can just go use UHID_CREATE2
> instead.
I do not know if anyone is still using UHID_CREATE with sendpage and
neither do you really. It is all about not breaking userspace without
good reason and here ensuring that we switch to USER_DS and then back
to whatever it was does not seem too hard.
>
> > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > + task_tgid_vnr(current), current->comm);
> > > > + ret = -EACCES;
> > > > + goto unlock;
> > > > + }
> > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > break;
> > > > case UHID_CREATE2:
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 22:46 ` Dmitry Torokhov
@ 2018-11-15 0:39 ` Andy Lutomirski
0 siblings, 0 replies; 25+ messages in thread
From: Andy Lutomirski @ 2018-11-15 0:39 UTC (permalink / raw)
To: Dmitry Torokhov
Cc: jannh, ebiggers, dh.herrmann, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, lkml, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, luto
> On Nov 14, 2018, at 2:46 PM, Dmitry Torokhov <dtor@google.com> wrote:
>
>> On Wed, Nov 14, 2018 at 2:38 PM Jann Horn <jannh@google.com> wrote:
>>
>>> On Wed, Nov 14, 2018 at 11:29 PM Dmitry Torokhov <dtor@google.com> wrote:
>>>> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
>>>>> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
>>>>> When a UHID_CREATE command is written to the uhid char device, a
>>>>> copy_from_user() is done from a user pointer embedded in the command.
>>>>> When the address limit is KERNEL_DS, e.g. as is the case during
>>>>> sys_sendfile(), this can read from kernel memory. Alternatively,
>>>>> information can be leaked from a setuid binary that is tricked to write
>>>>> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>>>>>
>>>>> No other commands in uhid_char_write() are affected by this bug and
>>>>> UHID_CREATE is marked as "obsolete", so apply the restriction to
>>>>> UHID_CREATE only rather than to uhid_char_write() entirely.
>> [...]
>>>>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> [...]
>>>>> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>>>>>
>>>>> switch (uhid->input_buf.type) {
>>>>> case UHID_CREATE:
>>>>> + /*
>>>>> + * 'struct uhid_create_req' contains a __user pointer which is
>>>>> + * copied from, so it's unsafe to allow this with elevated
>>>>> + * privileges (e.g. from a setuid binary) or via kernel_write().
>>>>> + */
>>>
>>> uhid is a privileged interface so we would go from root to less
>>> privileged (if at all). If non-privileged process can open uhid it can
>>> construct virtual keyboard and inject whatever keystrokes it wants.
>>>
>>> Also, instead of disallowing access, can we ensure that we switch back
>>> to USER_DS before trying to load data from the user pointer?
>>
>> Does that even make sense? You are using some deprecated legacy
>> interface; you interact with it by splicing a request from something
>> like a file or a pipe into the uhid device; but the request you're
>> splicing through contains a pointer into userspace memory? Do you know
>> of anyone who is actually doing that? If not, anyone who does want to
>> do this for some reason in the future can just go use UHID_CREATE2
>> instead.
>
> I do not know if anyone is still using UHID_CREATE with sendpage and
> neither do you really. It is all about not breaking userspace without
> good reason and here ensuring that we switch to USER_DS and then back
> to whatever it was does not seem too hard.
It’s about not breaking userspace *except as needed for security fixes*. User pointers in a write() payload is a big no-no.
Also, that f_cred hack is only barely enough. This isn’t just about attacking suid things — this bug allows poking at the address space of an unsuspecting process. So, if a privileged program opens a uhid fd and is then tricked into writing untrusted data to the same fd (which is supposed to be safe), then we have a problem. Fortunately, identically privileged programs usually still don’t share a cred pointer unless they came from the right place.
The real right fix is to remove UHID_CREATE outright. This is terminally broken.
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 22:28 ` Dmitry Torokhov
2018-11-14 22:37 ` Jann Horn
@ 2018-11-14 23:00 ` Eric Biggers
2018-11-14 23:20 ` Dmitry Torokhov
1 sibling, 1 reply; 25+ messages in thread
From: Eric Biggers @ 2018-11-14 23:00 UTC (permalink / raw)
To: Dmitry Torokhov
Cc: jannh, dh.herrmann, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, lkml, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, luto
Hi Dmitry,
On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> >
> > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > From: Eric Biggers <ebiggers@google.com>
> > >
> > > When a UHID_CREATE command is written to the uhid char device, a
> > > copy_from_user() is done from a user pointer embedded in the command.
> > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > information can be leaked from a setuid binary that is tricked to write
> > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > >
> > > No other commands in uhid_char_write() are affected by this bug and
> > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > >
> > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > helpers fault on kernel addresses"), allowing this bug to be found.
> > >
> > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > Cc: <stable@vger.kernel.org> # v3.6+
> > > Cc: Jann Horn <jannh@google.com>
> > > Cc: Andy Lutomirski <luto@kernel.org>
> > > Signed-off-by: Eric Biggers <ebiggers@google.com>
> >
> > Reviewed-by: Jann Horn <jannh@google.com>
> >
> > > ---
> > > drivers/hid/uhid.c | 12 ++++++++++++
> > > 1 file changed, 12 insertions(+)
> > >
> > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > index 3c55073136064..051639c09f728 100644
> > > --- a/drivers/hid/uhid.c
> > > +++ b/drivers/hid/uhid.c
> > > @@ -12,6 +12,7 @@
> > >
> > > #include <linux/atomic.h>
> > > #include <linux/compat.h>
> > > +#include <linux/cred.h>
> > > #include <linux/device.h>
> > > #include <linux/fs.h>
> > > #include <linux/hid.h>
> > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > >
> > > switch (uhid->input_buf.type) {
> > > case UHID_CREATE:
> > > + /*
> > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > + * copied from, so it's unsafe to allow this with elevated
> > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > + */
>
> uhid is a privileged interface so we would go from root to less
> privileged (if at all). If non-privileged process can open uhid it can
> construct virtual keyboard and inject whatever keystrokes it wants.
>
> Also, instead of disallowing access, can we ensure that we switch back
> to USER_DS before trying to load data from the user pointer?
>
Actually uhid doesn't have any capability checks, so it's up to userspace to
assign permissions to the device node. I think it's best not to make
assumptions about how the interface will be used and to be consistent with how
other ->write() methods in the kernel handle the misfeature where a __user
pointer in the write() or read() payload is dereferenced. Temporarily switching
to USER_DS would only avoid one of the two problems.
Do you think the proposed restrictions would actually break anything?
- Eric
> > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > + task_tgid_vnr(current), current->comm);
> > > + ret = -EACCES;
> > > + goto unlock;
> > > + }
> > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > break;
> > > case UHID_CREATE2:
> > > --
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 23:00 ` Eric Biggers
@ 2018-11-14 23:20 ` Dmitry Torokhov
2018-11-15 8:14 ` Benjamin Tissoires
0 siblings, 1 reply; 25+ messages in thread
From: Dmitry Torokhov @ 2018-11-14 23:20 UTC (permalink / raw)
To: ebiggers
Cc: jannh, dh.herrmann, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, lkml, syzkaller-bugs, Dmitry Vyukov,
syzbot+72473edc9bf4eb1c6556, stable, luto
On Wed, Nov 14, 2018 at 3:00 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> Hi Dmitry,
>
> On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> > >
> > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > >
> > > > From: Eric Biggers <ebiggers@google.com>
> > > >
> > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > information can be leaked from a setuid binary that is tricked to write
> > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > >
> > > > No other commands in uhid_char_write() are affected by this bug and
> > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > > >
> > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > > helpers fault on kernel addresses"), allowing this bug to be found.
> > > >
> > > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > > Cc: <stable@vger.kernel.org> # v3.6+
> > > > Cc: Jann Horn <jannh@google.com>
> > > > Cc: Andy Lutomirski <luto@kernel.org>
> > > > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > >
> > > Reviewed-by: Jann Horn <jannh@google.com>
> > >
> > > > ---
> > > > drivers/hid/uhid.c | 12 ++++++++++++
> > > > 1 file changed, 12 insertions(+)
> > > >
> > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > > index 3c55073136064..051639c09f728 100644
> > > > --- a/drivers/hid/uhid.c
> > > > +++ b/drivers/hid/uhid.c
> > > > @@ -12,6 +12,7 @@
> > > >
> > > > #include <linux/atomic.h>
> > > > #include <linux/compat.h>
> > > > +#include <linux/cred.h>
> > > > #include <linux/device.h>
> > > > #include <linux/fs.h>
> > > > #include <linux/hid.h>
> > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > >
> > > > switch (uhid->input_buf.type) {
> > > > case UHID_CREATE:
> > > > + /*
> > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > + * copied from, so it's unsafe to allow this with elevated
> > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > + */
> >
> > uhid is a privileged interface so we would go from root to less
> > privileged (if at all). If non-privileged process can open uhid it can
> > construct virtual keyboard and inject whatever keystrokes it wants.
> >
> > Also, instead of disallowing access, can we ensure that we switch back
> > to USER_DS before trying to load data from the user pointer?
> >
>
> Actually uhid doesn't have any capability checks, so it's up to userspace to
> assign permissions to the device node.
Yes. There are quite a few such instances where kernel does not bother
implementing superfluous checks and instead relies on userspace to
provide sane environment. IIRC nobody in the kernel enforces root
filesystem not be accessible to ordinary users, it is done with
generic permission checks.
> I think it's best not to make
> assumptions about how the interface will be used and to be consistent with how
> other ->write() methods in the kernel handle the misfeature where a __user
> pointer in the write() or read() payload is dereferenced.
I can see that you might want to check credentials, etc, if interface
can be accessed by unprivileged process, however is it a big no no for
uhid/userio/uinput devices.
> Temporarily switching
> to USER_DS would only avoid one of the two problems.
So because of the above there is only one problem. If your system
opened access to uhid to random processes you have much bigger
problems than exposing some data from a suid binary. You can spam "rm
-rf .; rm -rf /" though uhid if there is interactive session
somewhere.
>
> Do you think the proposed restrictions would actually break anything?
It would break if someone uses UHID_CREATE with sendpage. I do not
know if anyone does. If we were certain there are no users we'd simply
removed UHID_CREATE altogether.
>
> - Eric
>
> > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > + task_tgid_vnr(current), current->comm);
> > > > + ret = -EACCES;
> > > > + goto unlock;
> > > > + }
> > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > break;
> > > > case UHID_CREATE2:
> > > > --
Thanks.
--
Dmitry
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 23:20 ` Dmitry Torokhov
@ 2018-11-15 8:14 ` Benjamin Tissoires
2018-11-15 12:06 ` David Herrmann
0 siblings, 1 reply; 25+ messages in thread
From: Benjamin Tissoires @ 2018-11-15 8:14 UTC (permalink / raw)
To: Dmitry Torokhov
Cc: ebiggers, jannh, dh.herrmann, Jiri Kosina,
open list:HID CORE LAYER, lkml, syzkaller-bugs, dvyukov,
syzbot+72473edc9bf4eb1c6556, 3.8+,
luto
On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <dtor@google.com> wrote:
>
> On Wed, Nov 14, 2018 at 3:00 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > Hi Dmitry,
> >
> > On Wed, Nov 14, 2018 at 02:28:56PM -0800, 'Dmitry Torokhov' via syzkaller-bugs wrote:
> > > On Wed, Nov 14, 2018 at 2:05 PM Jann Horn <jannh@google.com> wrote:
> > > >
> > > > On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > > > >
> > > > > From: Eric Biggers <ebiggers@google.com>
> > > > >
> > > > > When a UHID_CREATE command is written to the uhid char device, a
> > > > > copy_from_user() is done from a user pointer embedded in the command.
> > > > > When the address limit is KERNEL_DS, e.g. as is the case during
> > > > > sys_sendfile(), this can read from kernel memory. Alternatively,
> > > > > information can be leaked from a setuid binary that is tricked to write
> > > > > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> > > > >
> > > > > No other commands in uhid_char_write() are affected by this bug and
> > > > > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > > > > UHID_CREATE only rather than to uhid_char_write() entirely.
> > > > >
> > > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > > > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > > > > helpers fault on kernel addresses"), allowing this bug to be found.
> > > > >
> > > > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> > > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > > > > Cc: <stable@vger.kernel.org> # v3.6+
> > > > > Cc: Jann Horn <jannh@google.com>
> > > > > Cc: Andy Lutomirski <luto@kernel.org>
> > > > > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > > >
> > > > Reviewed-by: Jann Horn <jannh@google.com>
> > > >
> > > > > ---
> > > > > drivers/hid/uhid.c | 12 ++++++++++++
> > > > > 1 file changed, 12 insertions(+)
> > > > >
> > > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> > > > > index 3c55073136064..051639c09f728 100644
> > > > > --- a/drivers/hid/uhid.c
> > > > > +++ b/drivers/hid/uhid.c
> > > > > @@ -12,6 +12,7 @@
> > > > >
> > > > > #include <linux/atomic.h>
> > > > > #include <linux/compat.h>
> > > > > +#include <linux/cred.h>
> > > > > #include <linux/device.h>
> > > > > #include <linux/fs.h>
> > > > > #include <linux/hid.h>
> > > > > @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
> > > > >
> > > > > switch (uhid->input_buf.type) {
> > > > > case UHID_CREATE:
> > > > > + /*
> > > > > + * 'struct uhid_create_req' contains a __user pointer which is
> > > > > + * copied from, so it's unsafe to allow this with elevated
> > > > > + * privileges (e.g. from a setuid binary) or via kernel_write().
> > > > > + */
> > >
> > > uhid is a privileged interface so we would go from root to less
> > > privileged (if at all). If non-privileged process can open uhid it can
> > > construct virtual keyboard and inject whatever keystrokes it wants.
> > >
> > > Also, instead of disallowing access, can we ensure that we switch back
> > > to USER_DS before trying to load data from the user pointer?
> > >
> >
> > Actually uhid doesn't have any capability checks, so it's up to userspace to
> > assign permissions to the device node.
>
> Yes. There are quite a few such instances where kernel does not bother
> implementing superfluous checks and instead relies on userspace to
> provide sane environment. IIRC nobody in the kernel enforces root
> filesystem not be accessible to ordinary users, it is done with
> generic permission checks.
>
> > I think it's best not to make
> > assumptions about how the interface will be used and to be consistent with how
> > other ->write() methods in the kernel handle the misfeature where a __user
> > pointer in the write() or read() payload is dereferenced.
>
> I can see that you might want to check credentials, etc, if interface
> can be accessed by unprivileged process, however is it a big no no for
> uhid/userio/uinput devices.
Yep, any sane distribution would restrict the permissions of
uhid/userio/uinput to only be accessed by root. If that ever changes,
there is already an issue with the system and it was compromised
either by a terribly dizzy sysadmin.
>
> > Temporarily switching
> > to USER_DS would only avoid one of the two problems.
>
> So because of the above there is only one problem. If your system
> opened access to uhid to random processes you have much bigger
> problems than exposing some data from a suid binary. You can spam "rm
> -rf .; rm -rf /" though uhid if there is interactive session
> somewhere.
>
> >
> > Do you think the proposed restrictions would actually break anything?
>
> It would break if someone uses UHID_CREATE with sendpage. I do not
> know if anyone does. If we were certain there are no users we'd simply
> removed UHID_CREATE altogether.
AFAICT, there are 2 users of uhid:
- bluez for BLE devices (using HID over GATT)
- hid-replay for debugging.
There might be a few other users that are making some user space
driver out of opencv, but I wouldn't expect those to be really
widespread.
IIRC, bluez uses UHID_CREATE2 and hid-replay should also (or ought to
be, but this can be easily fixed as I maintain the code and I am the
almost sole user).
Regarding the sendpage fix, doesn't David's patch fixes the issue
already (https://patchwork.kernel.org/patch/10682549/).
I am fine applying whatever patch that fixes the security issues, as
long as it doesn't break bluez nor the hid-replay uses I have for
debugging and regression testing.
Cheers,
Benjamin
>
> >
> > - Eric
> >
> > > > > + if (file->f_cred != current_cred() || uaccess_kernel()) {
> > > > > + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> > > > > + task_tgid_vnr(current), current->comm);
> > > > > + ret = -EACCES;
> > > > > + goto unlock;
> > > > > + }
> > > > > ret = uhid_dev_create(uhid, &uhid->input_buf);
> > > > > break;
> > > > > case UHID_CREATE2:
> > > > > --
>
> Thanks.
>
> --
> Dmitry
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-15 8:14 ` Benjamin Tissoires
@ 2018-11-15 12:06 ` David Herrmann
2018-11-15 14:50 ` Andy Lutomirski
0 siblings, 1 reply; 25+ messages in thread
From: David Herrmann @ 2018-11-15 12:06 UTC (permalink / raw)
To: Benjamin Tissoires
Cc: Dmitry Torokhov, ebiggers, jannh, Jiri Kosina,
open list:HID CORE LAYER, linux-kernel, syzkaller-bugs,
Dmitry Vyukov, syzbot+72473edc9bf4eb1c6556, stable,
Andy Lutomirski
Hey
On Thu, Nov 15, 2018 at 9:14 AM Benjamin Tissoires
<benjamin.tissoires@redhat.com> wrote:
>
> On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <dtor@google.com> wrote:
> > > I think it's best not to make
> > > assumptions about how the interface will be used and to be consistent with how
> > > other ->write() methods in the kernel handle the misfeature where a __user
> > > pointer in the write() or read() payload is dereferenced.
> >
> > I can see that you might want to check credentials, etc, if interface
> > can be accessed by unprivileged process, however is it a big no no for
> > uhid/userio/uinput devices.
>
> Yep, any sane distribution would restrict the permissions of
> uhid/userio/uinput to only be accessed by root. If that ever changes,
> there is already an issue with the system and it was compromised
> either by a terribly dizzy sysadmin.
UHID is safe to be used by a non-root user. This does not imply that
you should open up access to the world, but you are free to have a
dedicated group or user with access to uhid. I agree that in most
common desktop-scenarios you should not grant world-access to it,
though.
> >
> > > Temporarily switching
> > > to USER_DS would only avoid one of the two problems.
> >
> > So because of the above there is only one problem. If your system
> > opened access to uhid to random processes you have much bigger
> > problems than exposing some data from a suid binary. You can spam "rm
> > -rf .; rm -rf /" though uhid if there is interactive session
> > somewhere.
> >
> > >
> > > Do you think the proposed restrictions would actually break anything?
> >
> > It would break if someone uses UHID_CREATE with sendpage. I do not
> > know if anyone does. If we were certain there are no users we'd simply
> > removed UHID_CREATE altogether.
>
> AFAICT, there are 2 users of uhid:
> - bluez for BLE devices (using HID over GATT)
> - hid-replay for debugging.
There are several more (eg., android bt-broadcom), and UHID_CREATE is
actively used. Dropping support for it will break these use-cases.
Thanks
David
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-15 12:06 ` David Herrmann
@ 2018-11-15 14:50 ` Andy Lutomirski
0 siblings, 0 replies; 25+ messages in thread
From: Andy Lutomirski @ 2018-11-15 14:50 UTC (permalink / raw)
To: David Herrmann
Cc: Benjamin Tissoires, Dmitry Torokhov, ebiggers, jannh,
Jiri Kosina, open list:HID CORE LAYER, linux-kernel,
syzkaller-bugs, Dmitry Vyukov, syzbot+72473edc9bf4eb1c6556,
stable, Andy Lutomirski
> On Nov 15, 2018, at 4:06 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
>
> Hey
>
> On Thu, Nov 15, 2018 at 9:14 AM Benjamin Tissoires
> <benjamin.tissoires@redhat.com> wrote:
>>
>> On Thu, Nov 15, 2018 at 12:20 AM Dmitry Torokhov <dtor@google.com> wrote:
>>>> I think it's best not to make
>>>> assumptions about how the interface will be used and to be consistent with how
>>>> other ->write() methods in the kernel handle the misfeature where a __user
>>>> pointer in the write() or read() payload is dereferenced.
>>>
>>> I can see that you might want to check credentials, etc, if interface
>>> can be accessed by unprivileged process, however is it a big no no for
>>> uhid/userio/uinput devices.
>>
>> Yep, any sane distribution would restrict the permissions of
>> uhid/userio/uinput to only be accessed by root. If that ever changes,
>> there is already an issue with the system and it was compromised
>> either by a terribly dizzy sysadmin.
>
> UHID is safe to be used by a non-root user. This does not imply that
> you should open up access to the world, but you are free to have a
> dedicated group or user with access to uhid. I agree that in most
> common desktop-scenarios you should not grant world-access to it,
> though.
>
>>>
>>>> Temporarily switching
>>>> to USER_DS would only avoid one of the two problems.
>>>
>>> So because of the above there is only one problem. If your system
>>> opened access to uhid to random processes you have much bigger
>>> problems than exposing some data from a suid binary. You can spam "rm
>>> -rf .; rm -rf /" though uhid if there is interactive session
>>> somewhere.
>>>
>>>>
>>>> Do you think the proposed restrictions would actually break anything?
>>>
>>> It would break if someone uses UHID_CREATE with sendpage. I do not
>>> know if anyone does. If we were certain there are no users we'd simply
>>> removed UHID_CREATE altogether.
>>
>> AFAICT, there are 2 users of uhid:
>> - bluez for BLE devices (using HID over GATT)
>> - hid-replay for debugging.
>
> There are several more (eg., android bt-broadcom), and UHID_CREATE is
> actively used. Dropping support for it will break these use-cases.
>
>
Is the support story for these programs such that we could add a big scary warning and remove UHID_CREATE in a couple months?
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 21:55 ` [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges Eric Biggers
2018-11-14 22:04 ` Jann Horn
@ 2018-11-15 12:09 ` David Herrmann
2018-11-15 14:49 ` Andy Lutomirski
2018-11-19 12:52 ` Jiri Kosina
2 siblings, 1 reply; 25+ messages in thread
From: David Herrmann @ 2018-11-15 12:09 UTC (permalink / raw)
To: ebiggers
Cc: Jiri Kosina, Benjamin Tissoires, open list:HID CORE LAYER,
linux-kernel, syzkaller-bugs, Dmitry Vyukov, Dmitry Torokhov,
syzbot+72473edc9bf4eb1c6556, stable, jannh, Andy Lutomirski
Hi
On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <stable@vger.kernel.org> # v3.6+
> Cc: Jann Horn <jannh@google.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> drivers/hid/uhid.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 3c55073136064..051639c09f728 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -12,6 +12,7 @@
>
> #include <linux/atomic.h>
> #include <linux/compat.h>
> +#include <linux/cred.h>
> #include <linux/device.h>
> #include <linux/fs.h>
> #include <linux/hid.h>
> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>
> switch (uhid->input_buf.type) {
> case UHID_CREATE:
> + /*
> + * 'struct uhid_create_req' contains a __user pointer which is
> + * copied from, so it's unsafe to allow this with elevated
> + * privileges (e.g. from a setuid binary) or via kernel_write().
> + */
> + if (file->f_cred != current_cred() || uaccess_kernel()) {
I think `uaccess_kernel()` would be enough. UHID does not check any
credentials. If you believe this should be there nevertheless, feel
free to keep it. Either way:
Acked-by: David Herrmann <dh.herrmann@gmail.com>
Thanks
David
> + pr_err_once("UHID_CREATE from different security context by process %d (%s), this is not allowed.\n",
> + task_tgid_vnr(current), current->comm);
> + ret = -EACCES;
> + goto unlock;
> + }
> ret = uhid_dev_create(uhid, &uhid->input_buf);
> break;
> case UHID_CREATE2:
> --
> 2.19.1.930.g4563a0d9d0-goog
>
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-15 12:09 ` David Herrmann
@ 2018-11-15 14:49 ` Andy Lutomirski
0 siblings, 0 replies; 25+ messages in thread
From: Andy Lutomirski @ 2018-11-15 14:49 UTC (permalink / raw)
To: David Herrmann
Cc: ebiggers, Jiri Kosina, Benjamin Tissoires,
open list:HID CORE LAYER, linux-kernel, syzkaller-bugs,
Dmitry Vyukov, Dmitry Torokhov, syzbot+72473edc9bf4eb1c6556,
stable, jannh, Andy Lutomirski
> On Nov 15, 2018, at 4:09 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
>
> Hi
>
>> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@kernel.org> wrote:
>> From: Eric Biggers <ebiggers@google.com>
>>
>> When a UHID_CREATE command is written to the uhid char device, a
>> copy_from_user() is done from a user pointer embedded in the command.
>> When the address limit is KERNEL_DS, e.g. as is the case during
>> sys_sendfile(), this can read from kernel memory. Alternatively,
>> information can be leaked from a setuid binary that is tricked to write
>> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>>
>> No other commands in uhid_char_write() are affected by this bug and
>> UHID_CREATE is marked as "obsolete", so apply the restriction to
>> UHID_CREATE only rather than to uhid_char_write() entirely.
>>
>> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
>> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
>> helpers fault on kernel addresses"), allowing this bug to be found.
>>
>> Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
>> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
>> Cc: <stable@vger.kernel.org> # v3.6+
>> Cc: Jann Horn <jannh@google.com>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Signed-off-by: Eric Biggers <ebiggers@google.com>
>> ---
>> drivers/hid/uhid.c | 12 ++++++++++++
>> 1 file changed, 12 insertions(+)
>>
>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> index 3c55073136064..051639c09f728 100644
>> --- a/drivers/hid/uhid.c
>> +++ b/drivers/hid/uhid.c
>> @@ -12,6 +12,7 @@
>>
>> #include <linux/atomic.h>
>> #include <linux/compat.h>
>> +#include <linux/cred.h>
>> #include <linux/device.h>
>> #include <linux/fs.h>
>> #include <linux/hid.h>
>> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>>
>> switch (uhid->input_buf.type) {
>> case UHID_CREATE:
>> + /*
>> + * 'struct uhid_create_req' contains a __user pointer which is
>> + * copied from, so it's unsafe to allow this with elevated
>> + * privileges (e.g. from a setuid binary) or via kernel_write().
>> + */
>> + if (file->f_cred != current_cred() || uaccess_kernel()) {
>
> I think `uaccess_kernel()` would be enough. UHID does not check any
> credentials. If you believe this should be there nevertheless, feel
> free to keep it.
The free check is needed. Without it, consider what sudo >uhid_fd does. It doesn’t use sudo’s credentials, but it does read its address space.
Can this patch get a comment added?
> Either way:
>
> Acked-by: David Herrmann <dh.herrmann@gmail.com>
>
> Thanks
> David
^ permalink raw reply [flat|nested] 25+ messages in thread
* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-14 21:55 ` [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges Eric Biggers
2018-11-14 22:04 ` Jann Horn
2018-11-15 12:09 ` David Herrmann
@ 2018-11-19 12:52 ` Jiri Kosina
2018-11-19 13:21 ` David Herrmann
2 siblings, 1 reply; 25+ messages in thread
From: Jiri Kosina @ 2018-11-19 12:52 UTC (permalink / raw)
To: Eric Biggers
Cc: David Herrmann, Benjamin Tissoires, linux-input, linux-kernel,
syzkaller-bugs, Dmitry Vyukov, Dmitry Torokhov,
syzbot+72473edc9bf4eb1c6556, stable, Jann Horn, Andy Lutomirski,
David Herrmann
[ David added to CC ]
On Wed, 14 Nov 2018, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> When a UHID_CREATE command is written to the uhid char device, a
> copy_from_user() is done from a user pointer embedded in the command.
> When the address limit is KERNEL_DS, e.g. as is the case during
> sys_sendfile(), this can read from kernel memory. Alternatively,
> information can be leaked from a setuid binary that is tricked to write
> to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
>
> No other commands in uhid_char_write() are affected by this bug and
> UHID_CREATE is marked as "obsolete", so apply the restriction to
> UHID_CREATE only rather than to uhid_char_write() entirely.
>
> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> helpers fault on kernel addresses"), allowing this bug to be found.
>
> Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> Cc: <stable@vger.kernel.org> # v3.6+
> Cc: Jann Horn <jannh@google.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
Thanks for the patch. I however believe the fix below is more generic, and
would prefer taking that one in case noone sees any major flaw in that
I've overlooked. Thanks.
From: David Herrmann <dh.herrmann@gmail.com>
Subject: [PATCH] HID: uhid: prevent splice(2)
The kernel has a default implementation of splice(2) for writing from a
pipe into an arbitrary file. This behavior can be overriden by
providing an f_op.splice_write() callback.
Unfortunately, the default implementation of splice_write() takes page
by page from the source pipe, calls kmap() and passes the mapped page
as kernel-address to f_op.write(). Thus, it uses standard write(2) to
implement splice(2). However, since the page is kernel-mapped, they
have to `set_fs(get_ds())`. This is mostly fine, but UHID takes
command-streams through write(2), and thus it might interpret the data
taken as pointers. If called with KERNEL_DS, you can trick UHID to
allow kernel-space pointers as well.
As a simple fix, prevent splice(2) on UHID. It is unsecure, but it is
also non-functional. We need a linear mapping of the input in UHID, so
chunked input from splice(2) makes no sense, anyway.
Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
drivers/hid/uhid.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 3c5507313606..fefedc0b4dc6 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -753,6 +753,15 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
return ret ? ret : count;
}
+static ssize_t uhid_char_splice_write(struct pipe_inode_info *pipe,
+ struct file *out,
+ loff_t *ppos,
+ size_t len,
+ unsigned int flags)
+{
+ return -EOPNOTSUPP;
+}
+
static __poll_t uhid_char_poll(struct file *file, poll_table *wait)
{
struct uhid_device *uhid = file->private_data;
@@ -771,6 +780,7 @@ static const struct file_operations uhid_fops = {
.release = uhid_char_release,
.read = uhid_char_read,
.write = uhid_char_write,
+ .splice_write = uhid_char_splice_write,
.poll = uhid_char_poll,
.llseek = no_llseek,
};
--
Jiri Kosina
SUSE Labs
^ permalink raw reply related [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-19 12:52 ` Jiri Kosina
@ 2018-11-19 13:21 ` David Herrmann
2018-11-19 13:26 ` Jiri Kosina
0 siblings, 1 reply; 25+ messages in thread
From: David Herrmann @ 2018-11-19 13:21 UTC (permalink / raw)
To: Jiri Kosina
Cc: ebiggers, Benjamin Tissoires, open list:HID CORE LAYER,
linux-kernel, syzkaller-bugs, Dmitry Vyukov, Dmitry Torokhov,
syzbot+72473edc9bf4eb1c6556, stable, Jann Horn, Andy Lutomirski
Hey
On Mon, Nov 19, 2018 at 1:52 PM Jiri Kosina <jikos@kernel.org> wrote:
>
>
> [ David added to CC ]
>
> On Wed, 14 Nov 2018, Eric Biggers wrote:
>
> > From: Eric Biggers <ebiggers@google.com>
> >
> > When a UHID_CREATE command is written to the uhid char device, a
> > copy_from_user() is done from a user pointer embedded in the command.
> > When the address limit is KERNEL_DS, e.g. as is the case during
> > sys_sendfile(), this can read from kernel memory. Alternatively,
> > information can be leaked from a setuid binary that is tricked to write
> > to the file descriptor. Therefore, forbid UHID_CREATE in these cases.
> >
> > No other commands in uhid_char_write() are affected by this bug and
> > UHID_CREATE is marked as "obsolete", so apply the restriction to
> > UHID_CREATE only rather than to uhid_char_write() entirely.
> >
> > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
> > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
> > helpers fault on kernel addresses"), allowing this bug to be found.
> >
> > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
> > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
> > Cc: <stable@vger.kernel.org> # v3.6+
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
>
> Thanks for the patch. I however believe the fix below is more generic, and
> would prefer taking that one in case noone sees any major flaw in that
> I've overlooked. Thanks.
As Andy rightly pointed out, the credentials check is actually needed.
The scenario here is using a uhid-fd as stdout when executing a
setuid-program. This will possibly end up reading arbitrary memory
from the setuid program and use it as input for the hid-descriptor.
To my knowledge, this is a rather small attack surface. UHID is
usually a privileged interface, which in itself already gives you
ridiculous privileges. Furthermore, it only allows read-access if you
happen to be able to craft the message the setuid program writes
(which must be a valid user-space pointer, pointing to a valid hid
descriptor).
But people have been creative in the past, and they will find a way to
use this. So I do think Eric's patch here is the way to go.
Lastly, this only guards UHID_CREATE, which is already a deprecated
interface for several years. I don't think we can drop it any time
soon, but at least the other uhid interfaces should be safe.
Thanks
David
^ permalink raw reply [flat|nested] 25+ messages in thread* Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges
2018-11-19 13:21 ` David Herrmann
@ 2018-11-19 13:26 ` Jiri Kosina
0 siblings, 0 replies; 25+ messages in thread
From: Jiri Kosina @ 2018-11-19 13:26 UTC (permalink / raw)
To: David Herrmann
Cc: ebiggers, Benjamin Tissoires, open list:HID CORE LAYER,
linux-kernel, syzkaller-bugs, Dmitry Vyukov, Dmitry Torokhov,
syzbot+72473edc9bf4eb1c6556, stable, Jann Horn, Andy Lutomirski
On Mon, 19 Nov 2018, David Herrmann wrote:
> > Thanks for the patch. I however believe the fix below is more generic, and
> > would prefer taking that one in case noone sees any major flaw in that
> > I've overlooked. Thanks.
>
> As Andy rightly pointed out, the credentials check is actually needed.
> The scenario here is using a uhid-fd as stdout when executing a
> setuid-program. This will possibly end up reading arbitrary memory
> from the setuid program and use it as input for the hid-descriptor.
Ah, right, that's a very good point indeed; I've overlooked that (valid)
concern in the thread. Thanks for spotting that, Andy.
I've now applied Eric's patch. Thanks everybody,
--
Jiri Kosina
SUSE Labs
^ permalink raw reply [flat|nested] 25+ messages in thread