From b5a8b28915aad17b6f49ccacd6d3fef3890844d1 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Tue, 17 Apr 2012 10:58:02 -0400 Subject: [PATCH] fix buffer overflow in vfprintf on long writes to unbuffered files vfprintf temporarily swaps in a local buffer (for the duration of the operation) when the target stream is unbuffered; this both simplifies the implementation of functions like dprintf (they don't need their own buffers) and eliminates the pathologically bad performance of writing the formatted output with one or more write syscalls per formatting field. in cases like dprintf where we are dealing with a virgin FILE structure, everything worked correctly. however for long-lived files (like stderr), it's possible that the buffer bounds were already set for the internal zero-size buffer. on the next write, __stdio_write would pick up and use the new buffer provided by vfprintf, but the bound (wend) field was still pointing at the internal zero-size buffer's end. this in turn allowed unbounded writes to the temporary buffer. --- src/stdio/vfprintf.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c index 928c8c16..91c6b93a 100644 --- a/src/stdio/vfprintf.c +++ b/src/stdio/vfprintf.c @@ -652,8 +652,9 @@ int vfprintf(FILE *f, const char *fmt, va_list ap) FLOCK(f); if (!f->buf_size) { saved_buf = f->buf; - f->buf = internal_buf; + f->wpos = f->wbase = f->buf = internal_buf; f->buf_size = sizeof internal_buf; + f->wend = internal_buf + sizeof internal_buf; } ret = printf_core(f, fmt, &ap2, nl_arg, nl_type); if (saved_buf) { -- 2.20.1