The current vgacon's scroll up implementation uses a circural buffer in vgacon_scrollback_cur. It always advances tail to prepare it for the next write and caps it to zero if the next ->vc_size_row bytes won't fit.
But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new line might not fit to the end of the scrollback buffer in the next attempt to scroll. This leads to various crashes as vgacon_scrollback_update writes out of the buffer: BUG: unable to handle page fault for address: ffffc900001752a0 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page RIP: 0010:mutex_unlock+0x13/0x30 ... Call Trace: n_tty_write+0x1a0/0x4d0 tty_write+0x1a0/0x2e0
Or to KASAN reports: BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
So check whether the line fits in the buffer and wrap if needed. Do it before the loop as console_sem is held and ->vc_size_row cannot change during the execution of vgacon_scrollback_cur. If it does change, we need to ensure it does not change elsewhere, not here.
Also, we do not split the write of a line into chunks as that would break the consumers of the buffer. They expect ->cnt, ->tail and ->size to be in harmony and advanced by ->vc_size_row.
I found few reports of this in the past, some with patches included, some even 2 years old: https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X4... https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang...
This fixes CVE-2020-14331.
Big thanks to guys mentioned in the Reported-and-debugged-by lines below who actually found the root cause.
Signed-off-by: Jiri Slaby jslaby@suse.cz Reported-and-debugged-by: 张云海 zhangyunhai@nsfocus.com Reported-and-debugged-by: Yang Yingliang yangyingliang@huawei.com Reported-by: Kyungtae Kim kt0755@gmail.com Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Greg KH greg@kroah.com Cc: Solar Designer solar@openwall.com Cc: "Srivatsa S. Bhat" srivatsa@csail.mit.edu Cc: Anthony Liguori aliguori@amazon.com Cc: Security Officers security@kernel.org Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang yangyingliang@huawei.com Cc: Bartlomiej Zolnierkiewicz b.zolnierkie@samsung.com Cc: dri-devel@lists.freedesktop.org Cc: linux-fbdev@vger.kernel.org --- drivers/video/console/vgacon.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c index f0f3d573f848..13194bb246f8 100644 --- a/drivers/video/console/vgacon.c +++ b/drivers/video/console/vgacon.c @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
p = (void *) (c->vc_origin + t * c->vc_size_row);
+ /* vc_size_row might have changed by VT_RESIZE in the meantime */ + if ((vgacon_scrollback_cur->tail + c->vc_size_row) >= + vgacon_scrollback_cur->size) + vgacon_scrollback_cur->tail = 0; + while (count--) { scr_memcpyw(vgacon_scrollback_cur->data + vgacon_scrollback_cur->tail,
Hi All,
This patch dosen't fix the issue, the check should be in the loop.
The change of the VT sze is before vgacon_scrollback_update, not in the meantime.
Let's consider the following situation: suppose: vgacon_scrollback_cur->size is 65440 vgacon_scrollback_cur->tail is 64960 c->vc_size_row is 160 count is 5 Reset c->vc_size_row to 200 by VT_RESIZE, then call vgacon_scrollback_update. This will pass the check, since (vgacon_scrollback_cur->tail + c->vc_size_row) is 65160 which is less then vgacon_scrollback_cur->size(65440).
However, in the 3rd iteration of the loop, vgacon_scrollback_cur->tail is update to 65360, the memcpy will overflow.
To avoid overflow, the check should be if ((vgacon_scrollback_cur->tail + c->vc_size_row * count) >=
However, this will break the circular of the buffer, since all 5 lines will be copy at the beginning.
To avoid break circular, we have to detect if wrap occurs, use a loop to copy lines before wrap, reset vgacon_scrollback_cur->tail to 0, then use another loop to copy lines after wrap. Of course the 2 loop can be combine into 2 memcpy, that will be similar to Linus's patch.
Thus, I think the check should be in the loop. The 2 check in the loop seems to be redundancy, Zhang Xiao from Windriver suggest that the check after the memcpy can be remove. I think he was right, but not very sure. Thus, I suggest we discuss that too.
Regards, Yunhai Zhang / NSFOCUS Security Team
On 2020/7/29 15:02, Jiri Slaby wrote:
The current vgacon's scroll up implementation uses a circural buffer in vgacon_scrollback_cur. It always advances tail to prepare it for the next write and caps it to zero if the next ->vc_size_row bytes won't fit.
But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new line might not fit to the end of the scrollback buffer in the next attempt to scroll. This leads to various crashes as vgacon_scrollback_update writes out of the buffer: BUG: unable to handle page fault for address: ffffc900001752a0 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page RIP: 0010:mutex_unlock+0x13/0x30 ... Call Trace: n_tty_write+0x1a0/0x4d0 tty_write+0x1a0/0x2e0
Or to KASAN reports: BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
So check whether the line fits in the buffer and wrap if needed. Do it before the loop as console_sem is held and ->vc_size_row cannot change during the execution of vgacon_scrollback_cur. If it does change, we need to ensure it does not change elsewhere, not here.
Also, we do not split the write of a line into chunks as that would break the consumers of the buffer. They expect ->cnt, ->tail and ->size to be in harmony and advanced by ->vc_size_row.
I found few reports of this in the past, some with patches included, some even 2 years old: https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X4... https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang...
This fixes CVE-2020-14331.
Big thanks to guys mentioned in the Reported-and-debugged-by lines below who actually found the root cause.
Signed-off-by: Jiri Slaby jslaby@suse.cz Reported-and-debugged-by: 张云海 zhangyunhai@nsfocus.com Reported-and-debugged-by: Yang Yingliang yangyingliang@huawei.com Reported-by: Kyungtae Kim kt0755@gmail.com Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Greg KH greg@kroah.com Cc: Solar Designer solar@openwall.com Cc: "Srivatsa S. Bhat" srivatsa@csail.mit.edu Cc: Anthony Liguori aliguori@amazon.com Cc: Security Officers security@kernel.org Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang yangyingliang@huawei.com Cc: Bartlomiej Zolnierkiewicz b.zolnierkie@samsung.com Cc: dri-devel@lists.freedesktop.org Cc: linux-fbdev@vger.kernel.org
drivers/video/console/vgacon.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c index f0f3d573f848..13194bb246f8 100644 --- a/drivers/video/console/vgacon.c +++ b/drivers/video/console/vgacon.c @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
p = (void *) (c->vc_origin + t * c->vc_size_row);
- /* vc_size_row might have changed by VT_RESIZE in the meantime */
- if ((vgacon_scrollback_cur->tail + c->vc_size_row) >=
vgacon_scrollback_cur->size)vgacon_scrollback_cur->tail = 0;- while (count--) { scr_memcpyw(vgacon_scrollback_cur->data + vgacon_scrollback_cur->tail,
Hi,
On 29. 07. 20, 9:53, 张云海 wrote:
This patch dosen't fix the issue, the check should be in the loop.
The change of the VT sze is before vgacon_scrollback_update, not in the meantime.
Let's consider the following situation: suppose: vgacon_scrollback_cur->size is 65440 vgacon_scrollback_cur->tail is 64960 c->vc_size_row is 160 count is 5
Reset c->vc_size_row to 200 by VT_RESIZE, then call vgacon_scrollback_update.
This will pass the check, since (vgacon_scrollback_cur->tail + c->vc_size_row) is 65160 which is less then vgacon_scrollback_cur->size(65440).
However, in the 3rd iteration of the loop, vgacon_scrollback_cur->tail is update to 65360, the memcpy will overflow.
But the loop checks for the overflow: if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size) vgacon_scrollback_cur->tail = 0;
So the first 2 iterations would write to the end of the buffer and this 3rd one should have zeroed ->tail.
thanks,
On 2020/7/29 16:11, Jiri Slaby wrote:
But the loop checks for the overflow: if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size) vgacon_scrollback_cur->tail = 0;
So the first 2 iterations would write to the end of the buffer and this 3rd one should have zeroed ->tail.
In the 2nd iteration before the check: vgacon_scrollback_cur->tail is 65360 which is still less then vgacon_scrollback_cur->size(65440), so the ->tail won't be zeroed.
Then it gose to the 3rd iteration, overflow occurs.
Regards, Yunhai Zhang / NSFOCUS Security Team
On 29. 07. 20, 10:19, 张云海 wrote:
On 2020/7/29 16:11, Jiri Slaby wrote:
But the loop checks for the overflow: if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size) vgacon_scrollback_cur->tail = 0;
So the first 2 iterations would write to the end of the buffer and this 3rd one should have zeroed ->tail.
In the 2nd iteration before the check: vgacon_scrollback_cur->tail is 65360 which is still less then vgacon_scrollback_cur->size(65440), so the ->tail won't be zeroed.
Then it gose to the 3rd iteration, overflow occurs.
Ahh, I see now! So it must be triggered by CSI M instead. It allows for more than 1 in count. So this is PoC for this case: #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/ioctl.h> #include <fcntl.h>
int main(int argc, char** argv) { int fd = open("/dev/tty1", O_RDWR); unsigned short size[3] = {25, 200, 0}; ioctl(fd, 0x5609, size); // VT_RESIZE
write(fd, "\e[1;1H", 6); for (int i = 0; i < 30; i++) write(fd, "\e[10M", 5); }
It corrupts memory, so it crashes the kernel randomly. Even with my before-loop patch.
So now: could you resend your patch with improved commit message, add all those Ccs etc.? You can copy most of the Ccs from my patch verbatim.
I am also not sure the test I was pointing out on the top of this message would be of any use after the change. But maybe leave the code rest in peace.
thanks,
Hi Jiri,
On 7/29/20 9:02 AM, Jiri Slaby wrote:
The current vgacon's scroll up implementation uses a circural buffer in vgacon_scrollback_cur. It always advances tail to prepare it for the next write and caps it to zero if the next ->vc_size_row bytes won't fit.
But when we change the VT size (e.g. by VT_RESIZE) in the meantime, the new line might not fit to the end of the scrollback buffer in the next attempt to scroll. This leads to various crashes as vgacon_scrollback_update writes out of the buffer: BUG: unable to handle page fault for address: ffffc900001752a0 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page RIP: 0010:mutex_unlock+0x13/0x30 ... Call Trace: n_tty_write+0x1a0/0x4d0 tty_write+0x1a0/0x2e0
Or to KASAN reports: BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
So check whether the line fits in the buffer and wrap if needed. Do it before the loop as console_sem is held and ->vc_size_row cannot change during the execution of vgacon_scrollback_cur. If it does change, we need to ensure it does not change elsewhere, not here.
Also, we do not split the write of a line into chunks as that would break the consumers of the buffer. They expect ->cnt, ->tail and ->size to be in harmony and advanced by ->vc_size_row.
I found few reports of this in the past, some with patches included, some even 2 years old: https://lore.kernel.org/lkml/CAEAjamsJnG-=TSOwgRbbb3B9Z-PA63oWmNPoKYWQ=Z=+X4...
Sorry but I don't work on fixing fbdev/console KASAN/syzbot/etc. reports (-ENORESOURCES). This has been made official in the past.
I'm happy to review/apply patches though.
https://lore.kernel.org/lkml/1589336932-35508-1-git-send-email-yangyingliang...
This was the first time the patch for issue was submitted.
I tried to apply it to drm-misc but then I have noticed that it has not been posted to linux-fbdev / dri-devel MLs (so it was not possible to merge it using dim tool) and thus I've requested the author to resend it:
https://lore.kernel.org/lkml/62544bd9-e47d-e7f9-92f2-49b8dbb132c1@samsung.co...
which he did:
https://lore.kernel.org/lkml/20200713105730.550334-1-yangyingliang@huawei.co...
and the patch is currently under review period (to give people chance to comment on it) and in my "to apply if no objections" folder.
I see that your/Yunhai patch addresses the root source of the issue so I'll be happy to apply/ACK it instead of Yang's patch once the final version is posted.
Thank you for working on this.
Best regards, -- Bartlomiej Zolnierkiewicz Samsung R&D Institute Poland Samsung Electronics
This fixes CVE-2020-14331.
Big thanks to guys mentioned in the Reported-and-debugged-by lines below who actually found the root cause.
Signed-off-by: Jiri Slaby jslaby@suse.cz Reported-and-debugged-by: 张云海 zhangyunhai@nsfocus.com Reported-and-debugged-by: Yang Yingliang yangyingliang@huawei.com Reported-by: Kyungtae Kim kt0755@gmail.com Fixes: 15bdab959c9b ([PATCH] vgacon: Add support for soft scrollback) Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Greg KH greg@kroah.com Cc: Solar Designer solar@openwall.com Cc: "Srivatsa S. Bhat" srivatsa@csail.mit.edu Cc: Anthony Liguori aliguori@amazon.com Cc: Security Officers security@kernel.org Cc: linux-distros@vs.openwall.org Cc: Yang Yingliang yangyingliang@huawei.com Cc: Bartlomiej Zolnierkiewicz b.zolnierkie@samsung.com Cc: dri-devel@lists.freedesktop.org Cc: linux-fbdev@vger.kernel.org
drivers/video/console/vgacon.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c index f0f3d573f848..13194bb246f8 100644 --- a/drivers/video/console/vgacon.c +++ b/drivers/video/console/vgacon.c @@ -250,6 +250,11 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
p = (void *) (c->vc_origin + t * c->vc_size_row);
- /* vc_size_row might have changed by VT_RESIZE in the meantime */
- if ((vgacon_scrollback_cur->tail + c->vc_size_row) >=
vgacon_scrollback_cur->size)vgacon_scrollback_cur->tail = 0;- while (count--) { scr_memcpyw(vgacon_scrollback_cur->data + vgacon_scrollback_cur->tail,
dri-devel@lists.freedesktop.org
-
Bartlomiej Zolnierkiewicz -
Jiri Slaby -
Jiri Slaby -
张云海