| Message ID | 20191029182320.GA17569@mwanda (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fbdev: potential information leak in do_fb_ioctl() | expand |
On Tue, 2019-10-29 at 21:23 +0300, Dan Carpenter wrote: > The "fix" struct has a 2 byte hole after ->ywrapstep and the > "fix = info->fix;" assignment doesn't necessarily clear it. It depends > on the compiler. [] > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c [] > @@ -1109,6 +1109,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd, > ret = -EFAULT; > break; > case FBIOGET_FSCREENINFO: > + memset(&fix, 0, sizeof(fix)); > lock_fb_info(info); > fix = info->fix; > if (info->flags & FBINFO_HIDE_SMEM_START) Perhaps better to change the struct copy to a memcpy --- drivers/video/fbdev/core/fbmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c index e6a1c80..364699 100644 --- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -1110,7 +1110,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd, break; case FBIOGET_FSCREENINFO: lock_fb_info(info); - fix = info->fix; + memcpy(&fix, &info->fix, sizeof(fix)); if (info->flags & FBINFO_HIDE_SMEM_START) fix.smem_start = 0; unlock_fb_info(info);
Dan Carpenter <dan.carpenter@oracle.com> writes: > The "fix" struct has a 2 byte hole after ->ywrapstep and the > "fix = info->fix;" assignment doesn't necessarily clear it. It depends > on the compiler. > > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > --- > I have 13 more similar places to patch... I'm not totally sure I > understand all the issues involved. What I have done in a similar situation with struct siginfo, is that where the structure first appears I have initialized it with memset, and then field by field. Then when the structure is copied I copy the structure with memcpy. That ensures all of the bytes in the original structure are initialized and that all of the bytes are copied. The goal is to avoid memory that has values of the previous users of that memory region from leaking to userspace. Which depending on who the previous user of that memory region is could tell userspace information about what the kernel is doing that it should not be allowed to find out. I tried to trace through where "info" and thus presumably "info->fix" is coming from and only made it as far as register_framebuffer. Given that I suspect a local memset, and then a field by field copy right before copy_to_user might be a sound solution. But ick. That is a lot of fields to copy. Eric > drivers/video/fbdev/core/fbmem.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c > index 6f6fc785b545..b4ce6a28aed9 100644 > --- a/drivers/video/fbdev/core/fbmem.c > +++ b/drivers/video/fbdev/core/fbmem.c > @@ -1109,6 +1109,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd, > ret = -EFAULT; > break; > case FBIOGET_FSCREENINFO: > + memset(&fix, 0, sizeof(fix)); > lock_fb_info(info); > fix = info->fix; > if (info->flags & FBINFO_HIDE_SMEM_START)
On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote: > Dan Carpenter <dan.carpenter@oracle.com> writes: > > > The "fix" struct has a 2 byte hole after ->ywrapstep and the > > "fix = info->fix;" assignment doesn't necessarily clear it. It depends > > on the compiler. > > > > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > > --- > > I have 13 more similar places to patch... I'm not totally sure I > > understand all the issues involved. > > What I have done in a similar situation with struct siginfo, is that > where the structure first appears I have initialized it with memset, > and then field by field. > > Then when the structure is copied I copy the structure with memcpy. > > That ensures all of the bytes in the original structure are initialized > and that all of the bytes are copied. > > The goal is to avoid memory that has values of the previous users of > that memory region from leaking to userspace. Which depending on who > the previous user of that memory region is could tell userspace > information about what the kernel is doing that it should not be allowed > to find out. > > I tried to trace through where "info" and thus presumably "info->fix" is > coming from and only made it as far as register_framebuffer. Given > that I suspect a local memset, and then a field by field copy right > before copy_to_user might be a sound solution. But ick. That is a lot > of fields to copy. I know it might sound quite inefficient, but what about making struct fb_fix_screeninfo __packed? This doesn't solve other potential similar issues, but for this particular case it could be a reasonable and simple fix. -Andrea
Andrea Righi <andrea.righi@canonical.com> writes: > On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote: >> Dan Carpenter <dan.carpenter@oracle.com> writes: >> >> > The "fix" struct has a 2 byte hole after ->ywrapstep and the >> > "fix = info->fix;" assignment doesn't necessarily clear it. It depends >> > on the compiler. >> > >> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") >> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> >> > --- >> > I have 13 more similar places to patch... I'm not totally sure I >> > understand all the issues involved. >> >> What I have done in a similar situation with struct siginfo, is that >> where the structure first appears I have initialized it with memset, >> and then field by field. >> >> Then when the structure is copied I copy the structure with memcpy. >> >> That ensures all of the bytes in the original structure are initialized >> and that all of the bytes are copied. >> >> The goal is to avoid memory that has values of the previous users of >> that memory region from leaking to userspace. Which depending on who >> the previous user of that memory region is could tell userspace >> information about what the kernel is doing that it should not be allowed >> to find out. >> >> I tried to trace through where "info" and thus presumably "info->fix" is >> coming from and only made it as far as register_framebuffer. Given >> that I suspect a local memset, and then a field by field copy right >> before copy_to_user might be a sound solution. But ick. That is a lot >> of fields to copy. > > I know it might sound quite inefficient, but what about making struct > fb_fix_screeninfo __packed? > > This doesn't solve other potential similar issues, but for this > particular case it could be a reasonable and simple fix. It is part of the user space ABI. As such you can't move the fields. Eric
On Wed, Oct 30, 2019 at 02:26:21PM -0500, Eric W. Biederman wrote: > Andrea Righi <andrea.righi@canonical.com> writes: > > > On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote: > >> Dan Carpenter <dan.carpenter@oracle.com> writes: > >> > >> > The "fix" struct has a 2 byte hole after ->ywrapstep and the > >> > "fix = info->fix;" assignment doesn't necessarily clear it. It depends > >> > on the compiler. > >> > > >> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") > >> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > >> > --- > >> > I have 13 more similar places to patch... I'm not totally sure I > >> > understand all the issues involved. > >> > >> What I have done in a similar situation with struct siginfo, is that > >> where the structure first appears I have initialized it with memset, > >> and then field by field. > >> > >> Then when the structure is copied I copy the structure with memcpy. > >> > >> That ensures all of the bytes in the original structure are initialized > >> and that all of the bytes are copied. > >> > >> The goal is to avoid memory that has values of the previous users of > >> that memory region from leaking to userspace. Which depending on who > >> the previous user of that memory region is could tell userspace > >> information about what the kernel is doing that it should not be allowed > >> to find out. > >> > >> I tried to trace through where "info" and thus presumably "info->fix" is > >> coming from and only made it as far as register_framebuffer. Given > >> that I suspect a local memset, and then a field by field copy right > >> before copy_to_user might be a sound solution. But ick. That is a lot > >> of fields to copy. > > > > I know it might sound quite inefficient, but what about making struct > > fb_fix_screeninfo __packed? > > > > This doesn't solve other potential similar issues, but for this > > particular case it could be a reasonable and simple fix. > > It is part of the user space ABI. As such you can't move the fields. > > Eric Oh, that's right! Then memset() + memcpy() is probably the best option, since copying all those fields one by one looks quite ugly to me... -Andrea
On Wed, 2019-10-30 at 21:12 +0100, Andrea Righi wrote: > Then memset() + memcpy() is probably the best option, > since copying all those fields one by one looks quite ugly to me... A memset of an automatic before a memcpy to the same automatic is unnecessary.
Joe Perches <joe@perches.com> writes: > On Wed, 2019-10-30 at 21:12 +0100, Andrea Righi wrote: >> Then memset() + memcpy() is probably the best option, >> since copying all those fields one by one looks quite ugly to me... > > A memset of an automatic before a memcpy to the same > automatic is unnecessary. You still need to guarantee that all of the holes in the structure you are copying are initialized before you copy it. Otherwise you are just changing which unitialized memory that is being copied to userspace. Which is my concern with your very simple suggestion. Eric
On 10/29/19 8:02 PM, Eric W. Biederman wrote: > Dan Carpenter <dan.carpenter@oracle.com> writes: > >> The "fix" struct has a 2 byte hole after ->ywrapstep and the >> "fix = info->fix;" assignment doesn't necessarily clear it. It depends >> on the compiler. >> >> Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") >> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> >> --- >> I have 13 more similar places to patch... I'm not totally sure I >> understand all the issues involved. > > What I have done in a similar situation with struct siginfo, is that > where the structure first appears I have initialized it with memset, > and then field by field. > > Then when the structure is copied I copy the structure with memcpy. > > That ensures all of the bytes in the original structure are initialized > and that all of the bytes are copied. > > The goal is to avoid memory that has values of the previous users of > that memory region from leaking to userspace. Which depending on who > the previous user of that memory region is could tell userspace > information about what the kernel is doing that it should not be allowed > to find out. > > I tried to trace through where "info" and thus presumably "info->fix" is > coming from and only made it as far as register_framebuffer. Given "info" (and thus "info->fix") comes from framebuffer_alloc() (which is called by fbdev device drivers prior to registering "info" with register_framebuffer()). framebuffer_alloc() does kzalloc() on "info". Therefore shouldn't memcpy() (as suggested by Jeo Perches) be enough? Best regards, -- Bartlomiej Zolnierkiewicz Samsung R&D Institute Poland Samsung Electronics > that I suspect a local memset, and then a field by field copy right > before copy_to_user might be a sound solution. But ick. That is a lot > of fields to copy. > > > Eric > > > >> drivers/video/fbdev/core/fbmem.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c >> index 6f6fc785b545..b4ce6a28aed9 100644 >> --- a/drivers/video/fbdev/core/fbmem.c >> +++ b/drivers/video/fbdev/core/fbmem.c >> @@ -1109,6 +1109,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd, >> ret = -EFAULT; >> break; >> case FBIOGET_FSCREENINFO: >> + memset(&fix, 0, sizeof(fix)); >> lock_fb_info(info); >> fix = info->fix; >> if (info->flags & FBINFO_HIDE_SMEM_START)
On Fri, Jan 3, 2020 at 2:09 PM Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com> wrote: > On 10/29/19 8:02 PM, Eric W. Biederman wrote: > > > > The goal is to avoid memory that has values of the previous users of > > that memory region from leaking to userspace. Which depending on who > > the previous user of that memory region is could tell userspace > > information about what the kernel is doing that it should not be allowed > > to find out. > > > > I tried to trace through where "info" and thus presumably "info->fix" is > > coming from and only made it as far as register_framebuffer. Given > > "info" (and thus "info->fix") comes from framebuffer_alloc() (which is > called by fbdev device drivers prior to registering "info" with > register_framebuffer()). framebuffer_alloc() does kzalloc() on "info". > > Therefore shouldn't memcpy() (as suggested by Jeo Perches) be enough? Is it guaranteed that all drivers call framebuffer_alloc() rather than open-coding it somewhere? Here is a list of all files that call register_framebuffer() without first calling framebuffer_alloc: $ git grep -wl register_framebuffer | xargs grep -L framebuffer_alloc Documentation/fb/framebuffer.rst drivers/media/pci/ivtv/ivtvfb.c drivers/media/platform/vivid/vivid-osd.c drivers/video/fbdev/68328fb.c drivers/video/fbdev/acornfb.c drivers/video/fbdev/amba-clcd.c drivers/video/fbdev/atafb.c drivers/video/fbdev/au1100fb.c drivers/video/fbdev/controlfb.c drivers/video/fbdev/core/fbmem.c drivers/video/fbdev/cyber2000fb.c drivers/video/fbdev/fsl-diu-fb.c drivers/video/fbdev/g364fb.c drivers/video/fbdev/goldfishfb.c drivers/video/fbdev/hpfb.c drivers/video/fbdev/macfb.c drivers/video/fbdev/matrox/matroxfb_base.c drivers/video/fbdev/matrox/matroxfb_crtc2.c drivers/video/fbdev/maxinefb.c drivers/video/fbdev/ocfb.c drivers/video/fbdev/pxafb.c drivers/video/fbdev/sa1100fb.c drivers/video/fbdev/stifb.c drivers/video/fbdev/valkyriefb.c drivers/video/fbdev/vermilion/vermilion.c drivers/video/fbdev/vt8500lcdfb.c drivers/video/fbdev/wm8505fb.c drivers/video/fbdev/xilinxfb.c It's possible (even likely, the ones I looked at are fine) that they all correctly zero out the fb_info structure first, but it seems hard to guarantee, so Eric's suggestion would possibly still be the safer choice. Arnd
On 1/13/20 1:49 PM, Arnd Bergmann wrote: > On Fri, Jan 3, 2020 at 2:09 PM Bartlomiej Zolnierkiewicz > <b.zolnierkie@samsung.com> wrote: >> On 10/29/19 8:02 PM, Eric W. Biederman wrote: >>> >>> The goal is to avoid memory that has values of the previous users of >>> that memory region from leaking to userspace. Which depending on who >>> the previous user of that memory region is could tell userspace >>> information about what the kernel is doing that it should not be allowed >>> to find out. >>> >>> I tried to trace through where "info" and thus presumably "info->fix" is >>> coming from and only made it as far as register_framebuffer. Given >> >> "info" (and thus "info->fix") comes from framebuffer_alloc() (which is >> called by fbdev device drivers prior to registering "info" with >> register_framebuffer()). framebuffer_alloc() does kzalloc() on "info". >> >> Therefore shouldn't memcpy() (as suggested by Jeo Perches) be enough? > > Is it guaranteed that all drivers call framebuffer_alloc() rather than > open-coding it somewhere? > > Here is a list of all files that call register_framebuffer() without first > calling framebuffer_alloc: > > $ git grep -wl register_framebuffer | xargs grep -L framebuffer_alloc > Documentation/fb/framebuffer.rst > drivers/media/pci/ivtv/ivtvfb.c > drivers/media/platform/vivid/vivid-osd.c > drivers/video/fbdev/68328fb.c > drivers/video/fbdev/acornfb.c > drivers/video/fbdev/amba-clcd.c > drivers/video/fbdev/atafb.c > drivers/video/fbdev/au1100fb.c > drivers/video/fbdev/controlfb.c > drivers/video/fbdev/core/fbmem.c > drivers/video/fbdev/cyber2000fb.c > drivers/video/fbdev/fsl-diu-fb.c > drivers/video/fbdev/g364fb.c > drivers/video/fbdev/goldfishfb.c > drivers/video/fbdev/hpfb.c > drivers/video/fbdev/macfb.c > drivers/video/fbdev/matrox/matroxfb_base.c > drivers/video/fbdev/matrox/matroxfb_crtc2.c > drivers/video/fbdev/maxinefb.c > drivers/video/fbdev/ocfb.c > drivers/video/fbdev/pxafb.c > drivers/video/fbdev/sa1100fb.c > drivers/video/fbdev/stifb.c > drivers/video/fbdev/valkyriefb.c > drivers/video/fbdev/vermilion/vermilion.c > drivers/video/fbdev/vt8500lcdfb.c > drivers/video/fbdev/wm8505fb.c > drivers/video/fbdev/xilinxfb.c > > It's possible (even likely, the ones I looked at are fine) that they > all correctly > zero out the fb_info structure first, but it seems hard to guarantee, so > Eric's suggestion would possibly still be the safer choice. I've audited all above instances and they are all fine. They either use the fb_info structure embedded in a driver specific structure (which is zeroed out) or (in case of some m68k specific drivers) use a static fb_info instance. Since fbdev is closed for new drivers it should be now fine to use the simpler approach (just use memcpy()). Best regards, -- Bartlomiej Zolnierkiewicz Samsung R&D Institute Poland Samsung Electronics
On Wed, Jan 15, 2020 at 2:09 PM Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com> wrote: > > $ git grep -wl register_framebuffer | xargs grep -L framebuffer_alloc > > Documentation/fb/framebuffer.rst > > drivers/media/pci/ivtv/ivtvfb.c > > drivers/media/platform/vivid/vivid-osd.c > > drivers/video/fbdev/68328fb.c > > drivers/video/fbdev/acornfb.c > > drivers/video/fbdev/amba-clcd.c > > drivers/video/fbdev/atafb.c > > drivers/video/fbdev/au1100fb.c > > drivers/video/fbdev/controlfb.c > > drivers/video/fbdev/core/fbmem.c > > drivers/video/fbdev/cyber2000fb.c > > drivers/video/fbdev/fsl-diu-fb.c > > drivers/video/fbdev/g364fb.c > > drivers/video/fbdev/goldfishfb.c > > drivers/video/fbdev/hpfb.c > > drivers/video/fbdev/macfb.c > > drivers/video/fbdev/matrox/matroxfb_base.c > > drivers/video/fbdev/matrox/matroxfb_crtc2.c > > drivers/video/fbdev/maxinefb.c > > drivers/video/fbdev/ocfb.c > > drivers/video/fbdev/pxafb.c > > drivers/video/fbdev/sa1100fb.c > > drivers/video/fbdev/stifb.c > > drivers/video/fbdev/valkyriefb.c > > drivers/video/fbdev/vermilion/vermilion.c > > drivers/video/fbdev/vt8500lcdfb.c > > drivers/video/fbdev/wm8505fb.c > > drivers/video/fbdev/xilinxfb.c > > > > It's possible (even likely, the ones I looked at are fine) that they > > all correctly > > zero out the fb_info structure first, but it seems hard to guarantee, so > > Eric's suggestion would possibly still be the safer choice. > > I've audited all above instances and they are all fine. They either > use the fb_info structure embedded in a driver specific structure > (which is zeroed out) or (in case of some m68k specific drivers) use > a static fb_info instance. > > Since fbdev is closed for new drivers it should be now fine to use > the simpler approach (just use memcpy()). Yes, let's do that then. The complex approach seems more likely to introduce a bug than one of the existing drivers to stop initializing the structure correctly. Arnd
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c index 6f6fc785b545..b4ce6a28aed9 100644 --- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -1109,6 +1109,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd, ret = -EFAULT; break; case FBIOGET_FSCREENINFO: + memset(&fix, 0, sizeof(fix)); lock_fb_info(info); fix = info->fix; if (info->flags & FBINFO_HIDE_SMEM_START)
The "fix" struct has a 2 byte hole after ->ywrapstep and the "fix = info->fix;" assignment doesn't necessarily clear it. It depends on the compiler. Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held") Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> --- I have 13 more similar places to patch... I'm not totally sure I understand all the issues involved. drivers/video/fbdev/core/fbmem.c | 1 + 1 file changed, 1 insertion(+)