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vgacon: Fix for missing check in scrollback handling
commitebfdfeeae8
upstream. vgacon_scrollback_update() always leaves enbough room in the scrollback buffer for the next call, but if the console size changed that room might not actually be enough, and so we need to re-check. The check should be in the loop since vgacon_scrollback_cur->tail is updated in the loop and count may be more than 1 when triggered by CSI M, as Jiri's PoC: #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 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 This fixes CVE-2020-14331. Reported-by: 张云海 <zhangyunhai@nsfocus.com> Reported-by: Yang Yingliang <yangyingliang@huawei.com> Reported-by: Kyungtae Kim <kt0755@gmail.com> Fixes:15bdab959c
([PATCH] vgacon: Add support for soft scrollback) Cc: stable@vger.kernel.org Cc: linux-fbdev@vger.kernel.org Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Solar Designer <solar@openwall.com> Cc: "Srivatsa S. Bhat" <srivatsa@csail.mit.edu> Cc: Anthony Liguori <aliguori@amazon.com> Cc: Yang Yingliang <yangyingliang@huawei.com> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com> Cc: Jiri Slaby <jirislaby@kernel.org> Signed-off-by: Yunhai Zhang <zhangyunhai@nsfocus.com> Link: https://lore.kernel.org/r/9fb43895-ca91-9b07-ebfd-808cf854ca95@nsfocus.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -246,6 +246,10 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
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p = (void *) (c->vc_origin + t * c->vc_size_row);
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while (count--) {
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if ((vgacon_scrollback_cur->tail + c->vc_size_row) >
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vgacon_scrollback_cur->size)
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vgacon_scrollback_cur->tail = 0;
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scr_memcpyw(vgacon_scrollback_cur->data +
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vgacon_scrollback_cur->tail,
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p, c->vc_size_row);
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