| Message ID | 20250109-tun-v2-2-388d7d5a287a@daynix.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | tun: Unify vnet implementation and fill full vnet header | expand |
On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > tun used to simply advance iov_iter when it needs to pad virtio header, > which leaves the garbage in the buffer as is. This is especially > problematic when tun starts to allow enabling the hash reporting > feature; even if the feature is enabled, the packet may lack a hash > value and may contain a hole in the virtio header because the packet > arrived before the feature gets enabled or does not contain the > header fields to be hashed. If the hole is not filled with zero, it is > impossible to tell if the packet lacks a hash value. > > In theory, a user of tun can fill the buffer with zero before calling > read() to avoid such a problem, but leaving the garbage in the buffer is > awkward anyway so fill the buffer in tun. > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> But if the user did it, you have just overwritten his value, did you not? > --- > drivers/net/tun_vnet.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c > index fe842df9e9ef..ffb2186facd3 100644 > --- a/drivers/net/tun_vnet.c > +++ b/drivers/net/tun_vnet.c > @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, > if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) > return -EFAULT; > > - iov_iter_advance(iter, sz - sizeof(*hdr)); > + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) > + return -EFAULT; > > return 0; > } > > -- > 2.47.1
On 2025/01/09 16:31, Michael S. Tsirkin wrote: > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: >> tun used to simply advance iov_iter when it needs to pad virtio header, >> which leaves the garbage in the buffer as is. This is especially >> problematic when tun starts to allow enabling the hash reporting >> feature; even if the feature is enabled, the packet may lack a hash >> value and may contain a hole in the virtio header because the packet >> arrived before the feature gets enabled or does not contain the >> header fields to be hashed. If the hole is not filled with zero, it is >> impossible to tell if the packet lacks a hash value. >> >> In theory, a user of tun can fill the buffer with zero before calling >> read() to avoid such a problem, but leaving the garbage in the buffer is >> awkward anyway so fill the buffer in tun. >> >> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > > But if the user did it, you have just overwritten his value, > did you not? Yes. but that means the user expects some part of buffer is not filled after read() or recvmsg(). I'm a bit worried that not filling the buffer may break assumptions others (especially the filesystem and socket infrastructures in the kernel) may have. If we are really confident that it will not cause problems, this behavior can be opt-in based on a flag or we can just write some documentation warning userspace programmers to initialize the buffer. > >> --- >> drivers/net/tun_vnet.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c >> index fe842df9e9ef..ffb2186facd3 100644 >> --- a/drivers/net/tun_vnet.c >> +++ b/drivers/net/tun_vnet.c >> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, >> if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) >> return -EFAULT; >> >> - iov_iter_advance(iter, sz - sizeof(*hdr)); >> + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) >> + return -EFAULT; >> >> return 0; >> } >> >> -- >> 2.47.1 >
On Thu, Jan 09, 2025 at 02:31:37AM -0500, Michael S. Tsirkin wrote: > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > > tun used to simply advance iov_iter when it needs to pad virtio header, > > which leaves the garbage in the buffer as is. This is especially > > problematic when tun starts to allow enabling the hash reporting > > feature; even if the feature is enabled, the packet may lack a hash > > value and may contain a hole in the virtio header because the packet > > arrived before the feature gets enabled or does not contain the > > header fields to be hashed. If the hole is not filled with zero, it is > > impossible to tell if the packet lacks a hash value. > > > > In theory, a user of tun can fill the buffer with zero before calling > > read() to avoid such a problem, but leaving the garbage in the buffer is > > awkward anyway so fill the buffer in tun. > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > > But if the user did it, you have just overwritten his value, > did you not? To clearify, I mean if user pre-filled buffer with 1, you have now regressed it. Patch 3 fixes it back, but - not pretty. > > --- > > drivers/net/tun_vnet.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c > > index fe842df9e9ef..ffb2186facd3 100644 > > --- a/drivers/net/tun_vnet.c > > +++ b/drivers/net/tun_vnet.c > > @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, > > if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) > > return -EFAULT; > > > > - iov_iter_advance(iter, sz - sizeof(*hdr)); > > + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) > > + return -EFAULT; > > > > return 0; > > } > > > > -- > > 2.47.1
On Thu, Jan 09, 2025 at 04:41:50PM +0900, Akihiko Odaki wrote: > On 2025/01/09 16:31, Michael S. Tsirkin wrote: > > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > > > tun used to simply advance iov_iter when it needs to pad virtio header, > > > which leaves the garbage in the buffer as is. This is especially > > > problematic when tun starts to allow enabling the hash reporting > > > feature; even if the feature is enabled, the packet may lack a hash > > > value and may contain a hole in the virtio header because the packet > > > arrived before the feature gets enabled or does not contain the > > > header fields to be hashed. If the hole is not filled with zero, it is > > > impossible to tell if the packet lacks a hash value. > > > > > > In theory, a user of tun can fill the buffer with zero before calling > > > read() to avoid such a problem, but leaving the garbage in the buffer is > > > awkward anyway so fill the buffer in tun. > > > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > > > > But if the user did it, you have just overwritten his value, > > did you not? > > Yes. but that means the user expects some part of buffer is not filled after > read() or recvmsg(). I'm a bit worried that not filling the buffer may break > assumptions others (especially the filesystem and socket infrastructures in > the kernel) may have. > > If we are really confident that it will not cause problems, this behavior > can be opt-in based on a flag or we can just write some documentation > warning userspace programmers to initialize the buffer. It's been like this for years, I'd say we wait until we know there's a problem? > > > > > --- > > > drivers/net/tun_vnet.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c > > > index fe842df9e9ef..ffb2186facd3 100644 > > > --- a/drivers/net/tun_vnet.c > > > +++ b/drivers/net/tun_vnet.c > > > @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, > > > if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) > > > return -EFAULT; > > > - iov_iter_advance(iter, sz - sizeof(*hdr)); > > > + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) > > > + return -EFAULT; > > > return 0; > > > } > > > > > > -- > > > 2.47.1 > >
On 2025/01/09 16:43, Michael S. Tsirkin wrote: > On Thu, Jan 09, 2025 at 04:41:50PM +0900, Akihiko Odaki wrote: >> On 2025/01/09 16:31, Michael S. Tsirkin wrote: >>> On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: >>>> tun used to simply advance iov_iter when it needs to pad virtio header, >>>> which leaves the garbage in the buffer as is. This is especially >>>> problematic when tun starts to allow enabling the hash reporting >>>> feature; even if the feature is enabled, the packet may lack a hash >>>> value and may contain a hole in the virtio header because the packet >>>> arrived before the feature gets enabled or does not contain the >>>> header fields to be hashed. If the hole is not filled with zero, it is >>>> impossible to tell if the packet lacks a hash value. >>>> >>>> In theory, a user of tun can fill the buffer with zero before calling >>>> read() to avoid such a problem, but leaving the garbage in the buffer is >>>> awkward anyway so fill the buffer in tun. >>>> >>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> >>> >>> But if the user did it, you have just overwritten his value, >>> did you not? >> >> Yes. but that means the user expects some part of buffer is not filled after >> read() or recvmsg(). I'm a bit worried that not filling the buffer may break >> assumptions others (especially the filesystem and socket infrastructures in >> the kernel) may have. >> >> If we are really confident that it will not cause problems, this behavior >> can be opt-in based on a flag or we can just write some documentation >> warning userspace programmers to initialize the buffer. > > It's been like this for years, I'd say we wait until we know there's a problem? Perhaps we can just leave it as is. Let me ask filesystem and networking people: Is it OK to leave some part of buffer uninitialized with read_iter() or recvmsg()? > >>> >>>> --- >>>> drivers/net/tun_vnet.c | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c >>>> index fe842df9e9ef..ffb2186facd3 100644 >>>> --- a/drivers/net/tun_vnet.c >>>> +++ b/drivers/net/tun_vnet.c >>>> @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, >>>> if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) >>>> return -EFAULT; >>>> - iov_iter_advance(iter, sz - sizeof(*hdr)); >>>> + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) >>>> + return -EFAULT; >>>> return 0; >>>> } >>>> >>>> -- >>>> 2.47.1 >>> >
On Thu 09-01-25 18:36:52, Akihiko Odaki wrote: > On 2025/01/09 16:43, Michael S. Tsirkin wrote: > > On Thu, Jan 09, 2025 at 04:41:50PM +0900, Akihiko Odaki wrote: > > > On 2025/01/09 16:31, Michael S. Tsirkin wrote: > > > > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > > > > > tun used to simply advance iov_iter when it needs to pad virtio header, > > > > > which leaves the garbage in the buffer as is. This is especially > > > > > problematic when tun starts to allow enabling the hash reporting > > > > > feature; even if the feature is enabled, the packet may lack a hash > > > > > value and may contain a hole in the virtio header because the packet > > > > > arrived before the feature gets enabled or does not contain the > > > > > header fields to be hashed. If the hole is not filled with zero, it is > > > > > impossible to tell if the packet lacks a hash value. > > > > > > > > > > In theory, a user of tun can fill the buffer with zero before calling > > > > > read() to avoid such a problem, but leaving the garbage in the buffer is > > > > > awkward anyway so fill the buffer in tun. > > > > > > > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > > > > > > > > But if the user did it, you have just overwritten his value, > > > > did you not? > > > > > > Yes. but that means the user expects some part of buffer is not filled after > > > read() or recvmsg(). I'm a bit worried that not filling the buffer may break > > > assumptions others (especially the filesystem and socket infrastructures in > > > the kernel) may have. > > > > > > If we are really confident that it will not cause problems, this behavior > > > can be opt-in based on a flag or we can just write some documentation > > > warning userspace programmers to initialize the buffer. > > > > It's been like this for years, I'd say we wait until we know there's a problem? > > Perhaps we can just leave it as is. Let me ask filesystem and networking > people: > > Is it OK to leave some part of buffer uninitialized with read_iter() or > recvmsg()? I think that leaving part of the IO buffer within returned IO length uninitialized is a very bad practice and I'm not aware of any place in filesystem area that would do that. It makes life unnecessarily harder for userspace and also it is invitation for subtle information leaks (depending on who allocates the buffer and who then gets to read the results). So I think the patch makes sense. Honza
Akihiko Odaki wrote: > On 2025/01/09 16:31, Michael S. Tsirkin wrote: > > On Thu, Jan 09, 2025 at 03:58:44PM +0900, Akihiko Odaki wrote: > >> tun used to simply advance iov_iter when it needs to pad virtio header, > >> which leaves the garbage in the buffer as is. This is especially > >> problematic when tun starts to allow enabling the hash reporting > >> feature; even if the feature is enabled, the packet may lack a hash > >> value and may contain a hole in the virtio header because the packet > >> arrived before the feature gets enabled or does not contain the > >> header fields to be hashed. If the hole is not filled with zero, it is > >> impossible to tell if the packet lacks a hash value. Zero is a valid hash value, so cannot be used as an indication that hashing is inactive. > >> In theory, a user of tun can fill the buffer with zero before calling > >> read() to avoid such a problem, but leaving the garbage in the buffer is > >> awkward anyway so fill the buffer in tun. > >> > >> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> > > > > But if the user did it, you have just overwritten his value, > > did you not? > > Yes. but that means the user expects some part of buffer is not filled > after read() or recvmsg(). I'm a bit worried that not filling the buffer > may break assumptions others (especially the filesystem and socket > infrastructures in the kernel) may have. If this is user memory that is ignored by the kernel, just reflected back, then there is no need in general to zero it. There are many such instances, also in msg_control. If not zeroing leads to ambiguity with the new feature, that would be a reason to add it -- it is always safe to do so. > If we are really confident that it will not cause problems, this > behavior can be opt-in based on a flag or we can just write some > documentation warning userspace programmers to initialize the buffer.
diff --git a/drivers/net/tun_vnet.c b/drivers/net/tun_vnet.c index fe842df9e9ef..ffb2186facd3 100644 --- a/drivers/net/tun_vnet.c +++ b/drivers/net/tun_vnet.c @@ -138,7 +138,8 @@ int tun_vnet_hdr_put(int sz, struct iov_iter *iter, if (copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)) return -EFAULT; - iov_iter_advance(iter, sz - sizeof(*hdr)); + if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr)) + return -EFAULT; return 0; }
tun used to simply advance iov_iter when it needs to pad virtio header, which leaves the garbage in the buffer as is. This is especially problematic when tun starts to allow enabling the hash reporting feature; even if the feature is enabled, the packet may lack a hash value and may contain a hole in the virtio header because the packet arrived before the feature gets enabled or does not contain the header fields to be hashed. If the hole is not filled with zero, it is impossible to tell if the packet lacks a hash value. In theory, a user of tun can fill the buffer with zero before calling read() to avoid such a problem, but leaving the garbage in the buffer is awkward anyway so fill the buffer in tun. Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> --- drivers/net/tun_vnet.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)