From 52c3c3a59234a9725b5dcfe9729ed737d7843980 Mon Sep 17 00:00:00 2001 From: Jiri Slaby Date: Mon, 24 Aug 2020 11:54:25 +0200 Subject: [PATCH] Revert "vc_screen: extract vcs_read_buf_header" This reverts commit b1c32fcfadf5593ab7a63261cc8a5747c36e627e, because Syzkaller reports a use-after-free, a write in vcs_read: BUG: KASAN: use-after-free in vcs_read_buf drivers/tty/vt/vc_screen.c:357 [inline] BUG: KASAN: use-after-free in vcs_read+0xaa7/0xb40 drivers/tty/vt/vc_screen.c:449 Write of size 2 at addr ffff8880a8014000 by task syz-executor.5/16936 CPU: 1 PID: 16936 Comm: syz-executor.5 Not tainted 5.9.0-rc1-next-20200820-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: ... kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530 vcs_read_buf drivers/tty/vt/vc_screen.c:357 [inline] vcs_read+0xaa7/0xb40 drivers/tty/vt/vc_screen.c:449 There are two issues with the patch: 1) vcs_read rounds the 'count' *up* to an even number. So if we read odd bytes from the header (3 bytes in the reproducer), the second byte of a (2-byte/ushort) write to temporary con_buf won't fit. It is because with the patch applied, we only subtract the real number read (3 bytes) and not the whole header (4 bytes). 2) in this scenario, we perform unaligned accesses now: there are 2-byte/ushort writes to odd addresses. Due to the same reason as above. Revert this for now, re-think and retry later. Signed-off-by: Jiri Slaby Reported-by: syzbot+ad1f53726c3bd11180cb@syzkaller.appspotmail.com Fixes: b1c32fcfadf5 ("vc_screen: extract vcs_read_buf_header") Cc: akpm@linux-foundation.org Cc: nico@fluxnic.net Link: https://lore.kernel.org/r/20200824095425.4376-1-jslaby@suse.cz Signed-off-by: Greg Kroah-Hartman --- drivers/tty/vt/vc_screen.c | 31 +++++++++++++------------------ 1 file changed, 13 insertions(+), 18 deletions(-) diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c index 3e3a6a9..1850bac 100644 --- a/drivers/tty/vt/vc_screen.c +++ b/drivers/tty/vt/vc_screen.c @@ -293,22 +293,6 @@ static void vcs_read_buf_noattr(const struct vc_data *vc, char *con_buf, } } -static unsigned int vcs_read_buf_header(const struct vc_data *vc, char *con_buf, - unsigned int pos, unsigned int count) -{ - count = min(HEADER_SIZE - pos, count); - - /* clamp header values if they don't fit */ - con_buf[0] = min(vc->vc_rows, 0xFFu); - con_buf[1] = min(vc->vc_cols, 0xFFu); - getconsxy(vc, con_buf + 2); - - if (pos) - memmove(con_buf, con_buf + pos, count); - - return count; -} - static unsigned int vcs_read_buf(const struct vc_data *vc, char *con_buf, unsigned int pos, unsigned int count, bool viewed, unsigned int *skip) @@ -318,11 +302,22 @@ static unsigned int vcs_read_buf(const struct vc_data *vc, char *con_buf, unsigned int filled = count; if (pos < HEADER_SIZE) { - count -= vcs_read_buf_header(vc, con_buf, pos, count); + /* clamp header values if they don't fit */ + con_buf[0] = min(vc->vc_rows, 0xFFu); + con_buf[1] = min(vc->vc_cols, 0xFFu); + getconsxy(vc, con_buf + 2); + + *skip += pos; + count += pos; + if (count > CON_BUF_SIZE) { + count = CON_BUF_SIZE; + filled = count - pos; + } + /* Advance state pointers and move on. */ + count -= min(HEADER_SIZE, count); pos = HEADER_SIZE; con_buf += HEADER_SIZE; - /* If count >= 0, then pos is even... */ } else if (pos & 1) { /* -- 2.7.4