Fix double free in __printf_fp_l (bug 26214)

  __printf_fp_l has a double free bug in the case where it allocates
memory with malloc internally, then has an I/O error while outputting
trailing padding and tries to free that already-freed memory when the
error occurs.  This patch fixes this by setting the relevant pointer
to NULL after the first free (the only free of this pointer that isn't
immediately followed by returning from the function).

Tested for x86_64 and x86.
  

On 08/07/2020 15:01, Joseph Myers wrote:
> __printf_fp_l has a double free bug in the case where it allocates
> memory with malloc internally, then has an I/O error while outputting
> trailing padding and tries to free that already-freed memory when the
> error occurs.  This patch fixes this by setting the relevant pointer
> to NULL after the first free (the only free of this pointer that isn't
> immediately followed by returning from the function).
> 
> Tested for x86_64 and x86.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
> index 73bf0da296..dc3fd38a19 100644
> --- a/stdio-common/Makefile
> +++ b/stdio-common/Makefile
> @@ -67,7 +67,8 @@ tests := tstscanf test_rdwr test-popen tstgetln test-fseek \
>  	 tst-renameat2 tst-bz11319 tst-bz11319-fortify2 \
>  	 scanf14a scanf16a \
>  	 tst-printf-bz25691 \
> -	 tst-vfprintf-width-prec-alloc
> +	 tst-vfprintf-width-prec-alloc \
> +	 tst-printf-fp-free
>  
>  
>  test-srcs = tst-unbputc tst-printf tst-printfsz-islongdouble

Ok.

> @@ -78,11 +79,13 @@ tests-special += $(objpfx)tst-unbputc.out $(objpfx)tst-printf.out \
>  		 $(objpfx)tst-setvbuf1-cmp.out \
>  		 $(objpfx)tst-vfprintf-width-prec-mem.out \
>  		 $(objpfx)tst-printfsz-islongdouble.out \
> -		 $(objpfx)tst-printf-bz25691-mem.out
> +		 $(objpfx)tst-printf-bz25691-mem.out \
> +		 $(objpfx)tst-printf-fp-free-mem.out
>  generated += tst-printf-bz18872.c tst-printf-bz18872.mtrace \
>  	     tst-printf-bz18872-mem.out \
>  	     tst-vfprintf-width-prec.mtrace tst-vfprintf-width-prec-mem.out \
> -	     tst-printf-bz25691.mtrace tst-printf-bz25691-mem.out
> +	     tst-printf-bz25691.mtrace tst-printf-bz25691-mem.out \
> +	     tst-printf-fp-free.mtrace tst-printf-fp-free-mem.out
>  endif
>  
>  tests-special += $(objpfx)tst-errno-manual.out
> @@ -108,6 +111,8 @@ tst-vfprintf-width-prec-ENV = \
>    MALLOC_TRACE=$(objpfx)tst-vfprintf-width-prec.mtrace
>  tst-printf-bz25691-ENV = \
>    MALLOC_TRACE=$(objpfx)tst-printf-bz25691.mtrace
> +tst-printf-fp-free-ENV = \
> +  MALLOC_TRACE=$(objpfx)tst-printf-fp-free.mtrace
>  
>  $(objpfx)tst-unbputc.out: tst-unbputc.sh $(objpfx)tst-unbputc
>  	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)' > $@; \

Ok.

> diff --git a/stdio-common/printf_fp.c b/stdio-common/printf_fp.c
> index 9e0ce962f2..49c693575e 100644
> --- a/stdio-common/printf_fp.c
> +++ b/stdio-common/printf_fp.c
> @@ -1250,6 +1250,9 @@ __printf_fp_l (FILE *fp, locale_t loc,
>  	{
>  	  free (buffer);
>  	  free (wbuffer);
> +	  /* Avoid a double free if the subsequent PADN encounters an
> +	     I/O error.  */
> +	  wbuffer = NULL;
>  	}
>      }
>  

Ok.

> diff --git a/stdio-common/tst-printf-fp-free.c b/stdio-common/tst-printf-fp-free.c
> new file mode 100644
> index 0000000000..fea52248e0
> --- /dev/null
> +++ b/stdio-common/tst-printf-fp-free.c
> @@ -0,0 +1,37 @@
> +/* Test double free bug in __printf_fp_l (bug 26214).
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <mcheck.h>
> +#include <stdio.h>
> +#include <sys/resource.h>
> +#include <support/check.h>
> +
> +static int
> +do_test (void)
> +{
> +  mtrace ();
> +  FILE *fp = fopen ("/dev/full", "w");
> +  TEST_VERIFY_EXIT (fp != NULL);
> +  char buf[131072];
> +  TEST_VERIFY_EXIT (setvbuf (fp, buf, _IOFBF, sizeof buf) == 0);
> +  TEST_COMPARE (fprintf (fp, "%-1000000.65536f", 1.0), -1);
> +  fclose (fp);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> 

Ok.
  

* Joseph Myers:

> __printf_fp_l has a double free bug in the case where it allocates
> memory with malloc internally, then has an I/O error while outputting
> trailing padding and tries to free that already-freed memory when the
> error occurs.  This patch fixes this by setting the relevant pointer
> to NULL after the first free (the only free of this pointer that isn't
> immediately followed by returning from the function).

Same question here: Should we treat this as a security bug?

Thanks,
Florian