While developing KASAN for 64-bit book3s, I hit the following stack
over-read.
It occurs because the hypercall to put characters onto the terminal
takes 2 longs (128 bits/16 bytes) of characters at a time, and so
hvc_put_chars() would unconditionally copy 16 bytes from the argument
buffer, regardless of supplied length. However, udbg_hvc_putc() can
call hvc_put_chars() with a single-byte buffer, leading to the error.
==================================================================
BUG: KASAN: stack-out-of-bounds in hvc_put_chars+0xdc/0x110
Read of size 8 at addr c0000000023e7a90 by task swapper/0
CPU: 0 PID: 0 Comm: swapper Not tainted 5.2.0-rc2-next-20190528-02824-g048a6ab4835b #113
Call Trace:
dump_stack+0x104/0x154 (unreliable)
print_address_description+0xa0/0x30c
__kasan_report+0x20c/0x224
kasan_report+0x18/0x30
__asan_report_load8_noabort+0x24/0x40
hvc_put_chars+0xdc/0x110
hvterm_raw_put_chars+0x9c/0x110
udbg_hvc_putc+0x154/0x200
udbg_write+0xf0/0x240
console_unlock+0x868/0xd30
register_console+0x970/0xe90
register_early_udbg_console+0xf8/0x114
setup_arch+0x108/0x790
start_kernel+0x104/0x784
start_here_common+0x1c/0x534
Memory state around the buggy address:
c0000000023e7980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
c0000000023e7a00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
>c0000000023e7a80: f1 f1 01 f2 f2 f2 00 00 00 00 00 00 00 00 00 00
^
c0000000023e7b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
c0000000023e7b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================
Document that a 16-byte buffer is requred, and provide it in udbg.
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
934bda59f2