Re: [PATCH v3] seccomp: Add find_notification helper

[Nathan Chancellor <natechancellor@gmail.com>]

On Thu, Jun 18, 2020 at 05:44:14AM +0000, Sargun Dhillon wrote:
> On Wed, Jun 17, 2020 at 01:08:44PM -0700, Nathan Chancellor wrote:
> > On Mon, Jun 01, 2020 at 04:25:32AM -0700, Sargun Dhillon wrote:
> > > This adds a helper which can iterate through a seccomp_filter to
> > > find a notification matching an ID. It removes several replicated
> > > chunks of code.
> > > 
> > > Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> > > Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> > > Reviewed-by: Tycho Andersen <tycho@tycho.ws>
> > > Cc: Matt Denton <mpdenton@google.com>
> > > Cc: Kees Cook <keescook@google.com>,
> > > Cc: Jann Horn <jannh@google.com>,
> > > Cc: Robert Sesek <rsesek@google.com>,
> > > Cc: Chris Palmer <palmer@google.com>
> > > Cc: Christian Brauner <christian.brauner@ubuntu.com>
> > > Cc: Tycho Andersen <tycho@tycho.ws>
> > > ---
> > >  kernel/seccomp.c | 55 ++++++++++++++++++++++++------------------------
> > >  1 file changed, 28 insertions(+), 27 deletions(-)
> > > 
> > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > > index 55a6184f5990..cc6b47173a95 100644
> > > --- a/kernel/seccomp.c
> > > +++ b/kernel/seccomp.c
> > > @@ -41,6 +41,7 @@
> > >  #include <linux/tracehook.h>
> > >  #include <linux/uaccess.h>
> > >  #include <linux/anon_inodes.h>
> > > +#include <linux/lockdep.h>
> > >  
> > >  enum notify_state {
> > >  	SECCOMP_NOTIFY_INIT,
> > > @@ -1021,10 +1022,27 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
> > >  	return 0;
> > >  }
> > >  
> > > +/* must be called with notif_lock held */
> > > +static inline struct seccomp_knotif *
> > > +find_notification(struct seccomp_filter *filter, u64 id)
> > > +{
> > > +	struct seccomp_knotif *cur;
> > > +
> > > +	lockdep_assert_held(&filter->notify_lock);
> > > +
> > > +	list_for_each_entry(cur, &filter->notif->notifications, list) {
> > > +		if (cur->id == id)
> > > +			return cur;
> > > +	}
> > > +
> > > +	return NULL;
> > > +}
> > > +
> > > +
> > >  static long seccomp_notify_recv(struct seccomp_filter *filter,
> > >  				void __user *buf)
> > >  {
> > > -	struct seccomp_knotif *knotif = NULL, *cur;
> > > +	struct seccomp_knotif *knotif, *cur;
> > >  	struct seccomp_notif unotif;
> > >  	ssize_t ret;
> > >  
> > > @@ -1078,15 +1096,8 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
> > >  		 * may have died when we released the lock, so we need to make
> > >  		 * sure it's still around.
> > >  		 */
> > > -		knotif = NULL;
> > >  		mutex_lock(&filter->notify_lock);
> > > -		list_for_each_entry(cur, &filter->notif->notifications, list) {
> > > -			if (cur->id == unotif.id) {
> > > -				knotif = cur;
> > > -				break;
> > > -			}
> > > -		}
> > > -
> > > +		knotif = find_notification(filter, unotif.id);
> > >  		if (knotif) {
> > >  			knotif->state = SECCOMP_NOTIFY_INIT;
> > >  			up(&filter->notif->request);
> > > @@ -1101,7 +1112,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
> > >  				void __user *buf)
> > >  {
> > >  	struct seccomp_notif_resp resp = {};
> > > -	struct seccomp_knotif *knotif = NULL, *cur;
> > > +	struct seccomp_knotif *knotif;
> > >  	long ret;
> > >  
> > >  	if (copy_from_user(&resp, buf, sizeof(resp)))
> > > @@ -1118,13 +1129,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
> > >  	if (ret < 0)
> > >  		return ret;
> > >  
> > > -	list_for_each_entry(cur, &filter->notif->notifications, list) {
> > > -		if (cur->id == resp.id) {
> > > -			knotif = cur;
> > > -			break;
> > > -		}
> > > -	}
> > > -
> > > +	knotif = find_notification(filter, resp.id);
> > >  	if (!knotif) {
> > >  		ret = -ENOENT;
> > >  		goto out;
> > > @@ -1150,7 +1155,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
> > >  static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> > >  				    void __user *buf)
> > >  {
> > > -	struct seccomp_knotif *knotif = NULL;
> > 
> > I don't know that this should have been removed, clang now warns:
> > 
> > kernel/seccomp.c:1063:2: warning: variable 'knotif' is used uninitialized whenever 'for' loop exits because its condition is false [-Wsometimes-uninitialized]
> >         list_for_each_entry(cur, &filter->notif->notifications, list) {
> >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/list.h:602:7: note: expanded from macro 'list_for_each_entry'
> >              &pos->member != (head);                                    \
> >              ^~~~~~~~~~~~~~~~~~~~~~
> > kernel/seccomp.c:1075:7: note: uninitialized use occurs here
> >         if (!knotif) {
> >              ^~~~~~
> > kernel/seccomp.c:1063:2: note: remove the condition if it is always true
> >         list_for_each_entry(cur, &filter->notif->notifications, list) {
> >         ^
> > include/linux/list.h:602:7: note: expanded from macro 'list_for_each_entry'
> >              &pos->member != (head);                                    \
> >              ^
> > kernel/seccomp.c:1045:31: note: initialize the variable 'knotif' to silence this warning
> >         struct seccomp_knotif *knotif, *cur;
> >                                      ^
> >                                       = NULL
> > 1 warning generated.
> > 
> I'm curious as to how you got clang to generate this warning. I'm running with clang 10, and
> upon running with V=1, and adding -Wsometimes-uninitialized, I'm not seeing this warning.
> The following is the command called:
> /usr/bin/clang-10 -Wp,-MD,kernel/.seccomp.o.d  -nostdinc -isystem /usr/lib/llvm-10/lib/clang/10.0.0/include -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -Qunused-arguments -Wsometimes-uninitialized -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-format-security -std=gnu89 -no-integrated-as -Werror=unknown-warning-option -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387 -mstack-alignment=8 -mtune=generic -mno-red-zone -mcmodel=kernel -DCONFIG_X86_X32_ABI -Wno-sign-compare -fno-asynchronous-unwind-tables -mretpoline-external-thunk -fno-delete-null-pointer-checks -Wno-address-of-packed-member -O2 -Wframe-larger-than=1024 -fstack-protector -Wno-format-invalid-specifier -Wno-gnu -mno-global-merge -Wno-unused-const-variable -g -pg -mfentry -DCC_USING_FENTRY -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wno-array-bounds -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types -fmacro-prefix-map=./= -fcf-protection=none -Wno-initializer-overrides -Wno-format -Wno-sign-compare -Wno-format-zero-length -Wno-tautological-constant-out-of-range-compare    -DKBUILD_MODFILE='"kernel/seccomp"' -DKBUILD_BASENAME='"seccomp"' -DKBUILD_MODNAME='"seccomp"' -c -o kernel/seccomp.o kernel/seccomp.c

I saw it on a variety of configs but the one that is probably the
easiest to reproduce with is arm32 defconfig:

$ curl -LSs https://lore.kernel.org/lkml/20200601112532.150158-1-sargun@sargun.me/raw | git am

$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 O=out/arm distclean defconfig kernel/seccomp.o

Cheers,
Nathan

> > > +	struct seccomp_knotif *knotif;
> > >  	u64 id;
> > >  	long ret;
> > >  
> > > @@ -1161,16 +1166,12 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> > >  	if (ret < 0)
> > >  		return ret;
> > >  
> > > -	ret = -ENOENT;
> > > -	list_for_each_entry(knotif, &filter->notif->notifications, list) {
> > > -		if (knotif->id == id) {
> > > -			if (knotif->state == SECCOMP_NOTIFY_SENT)
> > > -				ret = 0;
> > > -			goto out;
> > > -		}
> > > -	}
> > > +	knotif = find_notification(filter, id);
> > > +	if (knotif && knotif->state == SECCOMP_NOTIFY_SENT)
> > > +		ret = 0;
> > > +	else
> > > +		ret = -ENOENT;
> > >  
> > > -out:
> > >  	mutex_unlock(&filter->notify_lock);
> > >  	return ret;
> > >  }
> > > -- 
> > > 2.25.1
> > > 
> > 
> > Cheers,
> > Nathan