Re: [PATCH] media: dvb: fix a missing-check bug
Hi Wenwen, On Mon, Oct 29, 2018 at 01:46:04PM -0500, Wenwen Wang wrote: > Hello, > > Can anyone confirm this bug? Thanks! > > Wenwen > > On Fri, Oct 19, 2018 at 9:12 AM Wenwen Wang wrote: > > > > In dvb_audio_write(), the first byte of the user-space buffer 'buf' is > > firstly copied and checked to see whether this is a TS packet, which always > > starts with 0x47 for synchronization purposes. If yes, ts_play() will be > > called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of > > 'buf', including the first byte, is copied again from the user space. > > However, after the copy, no check is re-enforced on the first byte of the > > copied data. Given that 'buf' is in the user space, a malicious user can > > race to change the first byte after the check in dvb_audio_write() but > > before the copy in ts_play(). Up to here your analysis makes sense. > > Through this way, the user can supply > > inconsistent code, which can cause undefined behavior of the kernel and > > introduce potential security risk. So how can this cause undefined behaviour? > > This patch adds a necessary check in ts_play() to make sure the first byte > > acquired in the second copy contains the expected value. Otherwise, an > > error code EINVAL will be returned. So what about the other case, if dvb_play() was called due to the first byte not being 0x47 and then swapped for 0x47? Sean > > > > Signed-off-by: Wenwen Wang > > --- > > drivers/media/pci/ttpci/av7110_av.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/drivers/media/pci/ttpci/av7110_av.c > > b/drivers/media/pci/ttpci/av7110_av.c > > index ef1bc17..1ff6062 100644 > > --- a/drivers/media/pci/ttpci/av7110_av.c > > +++ b/drivers/media/pci/ttpci/av7110_av.c > > @@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const > > char __user *buf, > > } > > if (copy_from_user(kb, buf, TS_SIZE)) > > return -EFAULT; > > + if (kb[0] != 0x47) > > + return -EINVAL; > > write_ts_to_decoder(av7110, type, kb, TS_SIZE); > > todo -= TS_SIZE; > > buf += TS_SIZE; > > -- > > 2.7.4 > >
Re: [PATCH] media: dvb: fix a missing-check bug
Hi Wenwen, On Mon, Oct 29, 2018 at 01:46:04PM -0500, Wenwen Wang wrote: > Hello, > > Can anyone confirm this bug? Thanks! > > Wenwen > > On Fri, Oct 19, 2018 at 9:12 AM Wenwen Wang wrote: > > > > In dvb_audio_write(), the first byte of the user-space buffer 'buf' is > > firstly copied and checked to see whether this is a TS packet, which always > > starts with 0x47 for synchronization purposes. If yes, ts_play() will be > > called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of > > 'buf', including the first byte, is copied again from the user space. > > However, after the copy, no check is re-enforced on the first byte of the > > copied data. Given that 'buf' is in the user space, a malicious user can > > race to change the first byte after the check in dvb_audio_write() but > > before the copy in ts_play(). Up to here your analysis makes sense. > > Through this way, the user can supply > > inconsistent code, which can cause undefined behavior of the kernel and > > introduce potential security risk. So how can this cause undefined behaviour? > > This patch adds a necessary check in ts_play() to make sure the first byte > > acquired in the second copy contains the expected value. Otherwise, an > > error code EINVAL will be returned. So what about the other case, if dvb_play() was called due to the first byte not being 0x47 and then swapped for 0x47? Sean > > > > Signed-off-by: Wenwen Wang > > --- > > drivers/media/pci/ttpci/av7110_av.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/drivers/media/pci/ttpci/av7110_av.c > > b/drivers/media/pci/ttpci/av7110_av.c > > index ef1bc17..1ff6062 100644 > > --- a/drivers/media/pci/ttpci/av7110_av.c > > +++ b/drivers/media/pci/ttpci/av7110_av.c > > @@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const > > char __user *buf, > > } > > if (copy_from_user(kb, buf, TS_SIZE)) > > return -EFAULT; > > + if (kb[0] != 0x47) > > + return -EINVAL; > > write_ts_to_decoder(av7110, type, kb, TS_SIZE); > > todo -= TS_SIZE; > > buf += TS_SIZE; > > -- > > 2.7.4 > >
Re: [PATCH] media: dvb: fix a missing-check bug
Hello, Can anyone confirm this bug? Thanks! Wenwen On Fri, Oct 19, 2018 at 9:12 AM Wenwen Wang wrote: > > In dvb_audio_write(), the first byte of the user-space buffer 'buf' is > firstly copied and checked to see whether this is a TS packet, which always > starts with 0x47 for synchronization purposes. If yes, ts_play() will be > called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of > 'buf', including the first byte, is copied again from the user space. > However, after the copy, no check is re-enforced on the first byte of the > copied data. Given that 'buf' is in the user space, a malicious user can > race to change the first byte after the check in dvb_audio_write() but > before the copy in ts_play(). Through this way, the user can supply > inconsistent code, which can cause undefined behavior of the kernel and > introduce potential security risk. > > This patch adds a necessary check in ts_play() to make sure the first byte > acquired in the second copy contains the expected value. Otherwise, an > error code EINVAL will be returned. > > Signed-off-by: Wenwen Wang > --- > drivers/media/pci/ttpci/av7110_av.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/drivers/media/pci/ttpci/av7110_av.c > b/drivers/media/pci/ttpci/av7110_av.c > index ef1bc17..1ff6062 100644 > --- a/drivers/media/pci/ttpci/av7110_av.c > +++ b/drivers/media/pci/ttpci/av7110_av.c > @@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const char > __user *buf, > } > if (copy_from_user(kb, buf, TS_SIZE)) > return -EFAULT; > + if (kb[0] != 0x47) > + return -EINVAL; > write_ts_to_decoder(av7110, type, kb, TS_SIZE); > todo -= TS_SIZE; > buf += TS_SIZE; > -- > 2.7.4 >
Re: [PATCH] media: dvb: fix a missing-check bug
Hello, Can anyone confirm this bug? Thanks! Wenwen On Fri, Oct 19, 2018 at 9:12 AM Wenwen Wang wrote: > > In dvb_audio_write(), the first byte of the user-space buffer 'buf' is > firstly copied and checked to see whether this is a TS packet, which always > starts with 0x47 for synchronization purposes. If yes, ts_play() will be > called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of > 'buf', including the first byte, is copied again from the user space. > However, after the copy, no check is re-enforced on the first byte of the > copied data. Given that 'buf' is in the user space, a malicious user can > race to change the first byte after the check in dvb_audio_write() but > before the copy in ts_play(). Through this way, the user can supply > inconsistent code, which can cause undefined behavior of the kernel and > introduce potential security risk. > > This patch adds a necessary check in ts_play() to make sure the first byte > acquired in the second copy contains the expected value. Otherwise, an > error code EINVAL will be returned. > > Signed-off-by: Wenwen Wang > --- > drivers/media/pci/ttpci/av7110_av.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/drivers/media/pci/ttpci/av7110_av.c > b/drivers/media/pci/ttpci/av7110_av.c > index ef1bc17..1ff6062 100644 > --- a/drivers/media/pci/ttpci/av7110_av.c > +++ b/drivers/media/pci/ttpci/av7110_av.c > @@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const char > __user *buf, > } > if (copy_from_user(kb, buf, TS_SIZE)) > return -EFAULT; > + if (kb[0] != 0x47) > + return -EINVAL; > write_ts_to_decoder(av7110, type, kb, TS_SIZE); > todo -= TS_SIZE; > buf += TS_SIZE; > -- > 2.7.4 >
