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a=ed25519; pk=tC3hNkJQTpNX/gLKxTNQKDmiQl6QjBNCGKJINqAdJsE= X-Developer-Signature: v=1; a=ed25519-sha256; t=1715021719; l=3808; i=justinstitt@google.com; s=20230717; h=from:subject:message-id; bh=eQ59GvPtmD0StGAqEgoE1xzghWc3vAliYszQLIkM+hw=; b=XVVKygfhbc/BJCZeqQmuwfYK+srJRDUiBofzc7JKHYMaXrvrFCIV5PoAd9QhGRY/XOgmGRvF0 xo/Ugu0sK3RAGr0ImfBUXBRnnyuGe13P17PZRP3P68ZL01qO4fSVZEy X-Mailer: b4 0.12.3 Message-ID: <20240506-b4-sio-scrollback-delta-v1-1-4164d162a2b8@google.com> Subject: [PATCH] tty: vt: saturate scrollback_delta to avoid overflow From: Justin Stitt To: Greg Kroah-Hartman , Jiri Slaby , Nathan Chancellor , Bill Wendling Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, llvm@lists.linux.dev, Justin Stitt Content-Type: text/plain; charset="utf-8" Using the signed overflow sanitizer with syzkaller produces this UBSAN report: [ 31.304043] ------------[ cut here ]------------ [ 31.304048] UBSAN: signed-integer-overflow in ../drivers/tty/vt/vt.c:309:19 [ 31.304055] -2147483648 + -1073741824 cannot be represented in type 'int' [ 31.304066] CPU: 1 PID: 3894 Comm: syz-executor Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1 [ 31.304073] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 31.304077] Call Trace: [ 31.304080] [ 31.304083] dump_stack_lvl+0x93/0xd0 [ 31.304177] handle_overflow+0x171/0x1b0 [ 31.304186] scrollfront+0xcb/0xd0 [ 31.304196] tioclinux+0x3cc/0x450 [ 31.304205] tty_ioctl+0x7fc/0xc00 [ 31.304212] ? __pfx_tty_ioctl+0x10/0x10 [ 31.304219] __se_sys_ioctl+0xe0/0x140 [ 31.304228] do_syscall_64+0xd7/0x1b0 [ 31.304236] ? arch_exit_to_user_mode_prepare+0x11/0x60 [ 31.304244] entry_SYSCALL_64_after_hwframe+0x6f/0x77 [ 31.304254] RIP: 0033:0x7fc3902ae539 [ 31.304263] Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 14 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 8 [ 31.304282] RSP: 002b:00007ffc8a457998 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 [ 31.304289] RAX: ffffffffffffffda RBX: 00007fc3903e2f80 RCX: 00007fc3902ae539 [ 31.304293] RDX: 0000000020000040 RSI: 000000000000541c RDI: 0000000000000003 [ 31.304297] RBP: 00007fc39030d496 R08: 0000000000000000 R09: 0000000000000000 [ 31.304300] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 [ 31.304304] R13: 0000000000000800 R14: 00007fc3903e2f80 R15: 00007fc3903e2f80 [ 31.304310] [ 31.304371] ---[ end trace ]--- This is caused by the scrollback_delta overflowing. Historically, the signed integer overflow sanitizer did not work in the kernel due to its interaction with `-fwrapv` but this has since been changed [1] in the newest version of Clang; It being re-enabled in the kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow sanitizer"). Note that it would be difficult to reproduce this bug in a non-fuzzing scenario as it requires inputting tons of scroll inputs via keyboard before the scheduled console callback has had a chance to update. Nonetheless, let's saturate scrollback_delta so it stays clamped to integer bounds without wrapping around. [1]: https://github.com/llvm/llvm-project/pull/82432 Closes: https://github.com/KSPP/linux/issues/351 Signed-off-by: Justin Stitt --- Note: I am using Kees' SIO tree as a base: https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=dev/v6.8-rc2/signed-overflow-sanitizer --- drivers/tty/vt/vt.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 9b5b98dfc8b4..b4768336868e 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -308,7 +308,14 @@ static inline void scrolldelta(int lines) /* FIXME */ /* scrolldelta needs some kind of consistency lock, but the BKL was and still is not protecting versus the scheduled back end */ - scrollback_delta += lines; + + /* saturate scrollback_delta so that it never wraps around */ + if (lines > 0 && unlikely(INT_MAX - lines < scrollback_delta)) + scrollback_delta = INT_MAX; + else if (lines < 0 && unlikely(INT_MIN - lines > scrollback_delta)) + scrollback_delta = INT_MIN; + else + scrollback_delta += lines; schedule_console_callback(); } --- base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc change-id: 20240506-b4-sio-scrollback-delta-dff9aabace26 Best regards, -- Justin Stitt