[PATCH 4.18 116/135] vsprintf: Fix off-by-one bug in bstr_printf() processing dereferenced pointers

Greg Kroah-Hartman
2018-10-16T17:15:48Z

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

commit 62165600ae73ebd76e2d9b992b36360408d570d8 upstream.

The functions vbin_printf() and bstr_printf() are used by trace_printk() to
try to keep the overhead down during printing. trace_printk() uses
vbin_printf() at the time of execution, as it only scans the fmt string to
record the printf values into the buffer, and then uses vbin_printf() to do
the conversions to print the string based on the format and the saved
values in the buffer.

This is an issue for dereferenced pointers, as before commit 841a915d20c7b,
the processing of the pointer could happen some time after the pointer value
was recorded (reading the trace buffer). This means the processing of the
value at a later time could show different results, or even crash the
system, if the pointer no longer existed.

Commit 841a915d20c7b addressed this by processing dereferenced pointers at
the time of execution and save the result in the ring buffer as a string.
The bstr_printf() would then treat these pointers as normal strings, and
print the value. But there was an off-by-one bug here, where after
processing the argument, it move the pointer only "strlen(arg)" which made
the arg pointer not point to the next argument in the ring buffer, but
instead point to the nul character of the last argument. This causes any
values after a dereferenced pointer to be corrupted.

Cc: stable@vger.kernel.org
Fixes: 841a915d20c7b ("vsprintf: Do not have bprintf dereference pointers")
Reported-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
lib/vsprintf.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2769,7 +2769,7 @@ int bstr_printf(char *buf, size_t size,
copy = end - str;
memcpy(str, args, copy);
str += len;
- args += len;
+ args += len + 1;
}
}
if (process)