diff mbox series

Remove most vfprintf width/precision-dependent allocations (bug 14231, bug 26211)

Message ID alpine.DEB.2.21.2007070029230.17840@digraph.polyomino.org.uk
State Committed
Commit 6caddd34bd7ffb5ac4f36c8e036eee100c2cc535
Headers show
Series Remove most vfprintf width/precision-dependent allocations (bug 14231, bug 26211) | expand

Commit Message

Joseph Myers July 7, 2020, 12:30 a.m. UTC
The vfprintf implementation (used for all printf-family functions)
contains complicated logic to allocate internal buffers of a size
depending on the width and precision used for a format, using either
malloc or alloca depending on that size, and with consequent checks
for size overflow and allocation failure.

As noted in bug 26211, the version of that logic used when '$' plus
argument number formats are in use is missing the overflow checks,
which can result in segfaults (quite possibly exploitable, I didn't
try to work that out) when the width or precision is in the range
0x7fffffe0 through 0x7fffffff (maybe smaller values as well in the
wprintf case on 32-bit systems, when the multiplication by sizeof
(CHAR_T) can overflow).

All that complicated logic in fact appears to be useless.  As far as I
can tell, there has been no need (outside the floating-point printf
code, which does its own allocations) for allocations depending on
width or precision since commit
3e95f6602b226e0de06aaff686dc47b282d7cc16 ("Remove limitation on size
of precision for integers", Sun Sep 12 21:23:32 1999 +0000).  Thus,
this patch removes that logic completely, thereby fixing both problems
with excessive allocations for large width and precision for
non-floating-point formats, and the problem with missing overflow
checks with such allocations.  Note that this does have the
consequence that width and precision up to INT_MAX are now allowed
where previously INT_MAX / sizeof (CHAR_T) - EXTSIZ or more would have
been rejected, so could potentially expose any other overflows where
the value would previously have been rejected by those removed checks.

I believe this completely fixes bugs 14231 and 26211.

Excessive allocations are still possible in the floating-point case
(bug 21127), as are other integer or buffer overflows (see bug 26201).
This does not address the cases where a precision larger than INT_MAX
(embedded in the format string) would be meaningful without printf's
return value overflowing (when it's used with a string format, or %g
without the '#' flag, so the actual output will be much smaller), as
mentioned in bug 17829 comment 8; using size_t internally for
precision to handle that case would be complicated by struct
printf_info being a public ABI.  Nor does it address the matter of an
INT_MIN width being negated (bug 17829 comment 7; the same logic
appears a second time in the file as well, in the form of multiplying
by -1).  There may be other sources of memory allocations with malloc
in printf functions as well (bug 24988, bug 16060).  From inspection,
I think there are also integer overflows in two copies of "if ((width
-= len) < 0)" logic (where width is int, len is size_t and a very long
string could result in spurious padding being output on a 32-bit
system before printf overflows the count of output characters).

Tested for x86-64 and x86.

Comments

Andreas Schwab July 7, 2020, 7:31 a.m. UTC | #1
On Jul 07 2020, Joseph Myers wrote:

> All that complicated logic in fact appears to be useless.  As far as I
> can tell, there has been no need (outside the floating-point printf
> code, which does its own allocations) for allocations depending on
> width or precision since commit
> 3e95f6602b226e0de06aaff686dc47b282d7cc16 ("Remove limitation on size
> of precision for integers", Sun Sep 12 21:23:32 1999 +0000).

This is surprising, but I agree.

LGTM.

Andreas.
diff mbox series

Patch

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 0a8d66b846..1333203046 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -66,7 +66,8 @@  tests := tstscanf test_rdwr test-popen tstgetln test-fseek \
 	 tst-scanf-round \
 	 tst-renameat2 tst-bz11319 tst-bz11319-fortify2 \
 	 scanf14a scanf16a \
-	 tst-printf-bz25691
+	 tst-printf-bz25691 \
+	 tst-vfprintf-width-prec-alloc
 
 
 test-srcs = tst-unbputc tst-printf tst-printfsz-islongdouble
diff --git a/stdio-common/bug22.c b/stdio-common/bug22.c
index b3d48eb8e1..029b549941 100644
--- a/stdio-common/bug22.c
+++ b/stdio-common/bug22.c
@@ -57,7 +57,7 @@  do_test (void)
 
   ret = fprintf (fp, "%." SN3 "d", 1);
   printf ("ret = %d\n", ret);
-  if (ret != -1 || errno != EOVERFLOW)
+  if (ret != N3)
 	  return 1;
 
   /* GCC 9 warns about output of more than INT_MAX characters; this is
diff --git a/stdio-common/tst-vfprintf-width-prec-alloc.c b/stdio-common/tst-vfprintf-width-prec-alloc.c
new file mode 100644
index 0000000000..0a74b53a33
--- /dev/null
+++ b/stdio-common/tst-vfprintf-width-prec-alloc.c
@@ -0,0 +1,41 @@ 
+/* Test large width or precision does not involve large allocation.
+   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 <stdio.h>
+#include <sys/resource.h>
+#include <support/check.h>
+
+char test_string[] = "test";
+
+static int
+do_test (void)
+{
+  struct rlimit limit;
+  TEST_VERIFY_EXIT (getrlimit (RLIMIT_AS, &limit) == 0);
+  limit.rlim_cur = 200 * 1024 * 1024;
+  TEST_VERIFY_EXIT (setrlimit (RLIMIT_AS, &limit) == 0);
+  FILE *fp = fopen ("/dev/null", "w");
+  TEST_VERIFY_EXIT (fp != NULL);
+  TEST_COMPARE (fprintf (fp, "%1000000000d", 1), 1000000000);
+  TEST_COMPARE (fprintf (fp, "%.1000000000s", test_string), 4);
+  TEST_COMPARE (fprintf (fp, "%1000000000d %1000000000d", 1, 2), 2000000001);
+  TEST_COMPARE (fprintf (fp, "%2$.*1$s", 0x7fffffff, test_string), 4);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
index 7cb9333c53..e01c319b19 100644
--- a/stdio-common/vfprintf-internal.c
+++ b/stdio-common/vfprintf-internal.c
@@ -46,10 +46,6 @@ 
 #include <wctype.h>
 #endif
 
-/* In some cases we need extra space for all the output which is not
-   counted in the width of the string. We assume 32 characters is
-   enough.  */
-#define EXTSIZ		32
 #define ARGCHECK(S, Format) \
   do									      \
     {									      \
@@ -1343,7 +1339,6 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
 
   /* Buffer intermediate results.  */
   CHAR_T work_buffer[WORK_BUFFER_SIZE];
-  CHAR_T *workstart = NULL;
   CHAR_T *workend;
 
   /* We have to save the original argument pointer.  */
@@ -1452,7 +1447,6 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
       UCHAR_T pad = L_(' ');/* Padding character.  */
       CHAR_T spec;
 
-      workstart = NULL;
       workend = work_buffer + WORK_BUFFER_SIZE;
 
       /* Get current character in format string.  */
@@ -1544,31 +1538,6 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
 	    pad = L_(' ');
 	    left = 1;
 	  }
-
-	if (__glibc_unlikely (width >= INT_MAX / sizeof (CHAR_T) - EXTSIZ))
-	  {
-	    __set_errno (EOVERFLOW);
-	    done = -1;
-	    goto all_done;
-	  }
-
-	if (width >= WORK_BUFFER_SIZE - EXTSIZ)
-	  {
-	    /* We have to use a special buffer.  */
-	    size_t needed = ((size_t) width + EXTSIZ) * sizeof (CHAR_T);
-	    if (__libc_use_alloca (needed))
-	      workend = (CHAR_T *) alloca (needed) + width + EXTSIZ;
-	    else
-	      {
-		workstart = (CHAR_T *) malloc (needed);
-		if (workstart == NULL)
-		  {
-		    done = -1;
-		    goto all_done;
-		  }
-		workend = workstart + width + EXTSIZ;
-	      }
-	  }
       }
       JUMP (*f, step1_jumps);
 
@@ -1576,31 +1545,13 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
     LABEL (width):
       width = read_int (&f);
 
-      if (__glibc_unlikely (width == -1
-			    || width >= INT_MAX / sizeof (CHAR_T) - EXTSIZ))
+      if (__glibc_unlikely (width == -1))
 	{
 	  __set_errno (EOVERFLOW);
 	  done = -1;
 	  goto all_done;
 	}
 
-      if (width >= WORK_BUFFER_SIZE - EXTSIZ)
-	{
-	  /* We have to use a special buffer.  */
-	  size_t needed = ((size_t) width + EXTSIZ) * sizeof (CHAR_T);
-	  if (__libc_use_alloca (needed))
-	    workend = (CHAR_T *) alloca (needed) + width + EXTSIZ;
-	  else
-	    {
-	      workstart = (CHAR_T *) malloc (needed);
-	      if (workstart == NULL)
-		{
-		  done = -1;
-		  goto all_done;
-		}
-	      workend = workstart + width + EXTSIZ;
-	    }
-	}
       if (*f == L_('$'))
 	/* Oh, oh.  The argument comes from a positional parameter.  */
 	goto do_positional;
@@ -1649,34 +1600,6 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
 	}
       else
 	prec = 0;
-      if (prec > width && prec > WORK_BUFFER_SIZE - EXTSIZ)
-	{
-	  /* Deallocate any previously allocated buffer because it is
-	     too small.  */
-	  if (__glibc_unlikely (workstart != NULL))
-	    free (workstart);
-	  workstart = NULL;
-	  if (__glibc_unlikely (prec >= INT_MAX / sizeof (CHAR_T) - EXTSIZ))
-	    {
-	      __set_errno (EOVERFLOW);
-	      done = -1;
-	      goto all_done;
-	    }
-	  size_t needed = ((size_t) prec + EXTSIZ) * sizeof (CHAR_T);
-
-	  if (__libc_use_alloca (needed))
-	    workend = (CHAR_T *) alloca (needed) + prec + EXTSIZ;
-	  else
-	    {
-	      workstart = (CHAR_T *) malloc (needed);
-	      if (workstart == NULL)
-		{
-		  done = -1;
-		  goto all_done;
-		}
-	      workend = workstart + prec + EXTSIZ;
-	    }
-	}
       JUMP (*f, step2_jumps);
 
       /* Process 'h' modifier.  There might another 'h' following.  */
@@ -1740,10 +1663,6 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
       /* The format is correctly handled.  */
       ++nspecs_done;
 
-      if (__glibc_unlikely (workstart != NULL))
-	free (workstart);
-      workstart = NULL;
-
       /* Look for next format specifier.  */
 #ifdef COMPILE_WPRINTF
       f = __find_specwc ((end_of_spec = ++f));
@@ -1761,18 +1680,11 @@  vfprintf (FILE *s, const CHAR_T *format, va_list ap, unsigned int mode_flags)
 
   /* Hand off processing for positional parameters.  */
 do_positional:
-  if (__glibc_unlikely (workstart != NULL))
-    {
-      free (workstart);
-      workstart = NULL;
-    }
   done = printf_positional (s, format, readonly_format, ap, &ap_save,
 			    done, nspecs_done, lead_str_end, work_buffer,
 			    save_errno, grouping, thousands_sep, mode_flags);
 
  all_done:
-  if (__glibc_unlikely (workstart != NULL))
-    free (workstart);
   /* Unlock the stream.  */
   _IO_funlockfile (s);
   _IO_cleanup_region_end (0);
@@ -1816,8 +1728,6 @@  printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
   /* Just a counter.  */
   size_t cnt;
 
-  CHAR_T *workstart = NULL;
-
   if (grouping == (const char *) -1)
     {
 #ifdef COMPILE_WPRINTF
@@ -2010,7 +1920,6 @@  printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
       char pad = specs[nspecs_done].info.pad;
       CHAR_T spec = specs[nspecs_done].info.spec;
 
-      workstart = NULL;
       CHAR_T *workend = work_buffer + WORK_BUFFER_SIZE;
 
       /* Fill in last information.  */
@@ -2044,27 +1953,6 @@  printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
 	  prec = specs[nspecs_done].info.prec;
 	}
 
-      /* Maybe the buffer is too small.  */
-      if (MAX (prec, width) + EXTSIZ > WORK_BUFFER_SIZE)
-	{
-	  if (__libc_use_alloca ((MAX (prec, width) + EXTSIZ)
-				 * sizeof (CHAR_T)))
-	    workend = ((CHAR_T *) alloca ((MAX (prec, width) + EXTSIZ)
-					  * sizeof (CHAR_T))
-		       + (MAX (prec, width) + EXTSIZ));
-	  else
-	    {
-	      workstart = (CHAR_T *) malloc ((MAX (prec, width) + EXTSIZ)
-					     * sizeof (CHAR_T));
-	      if (workstart == NULL)
-		{
-		  done = -1;
-		  goto all_done;
-		}
-	      workend = workstart + (MAX (prec, width) + EXTSIZ);
-	    }
-	}
-
       /* Process format specifiers.  */
       while (1)
 	{
@@ -2138,18 +2026,12 @@  printf_positional (FILE *s, const CHAR_T *format, int readonly_format,
 	  break;
 	}
 
-      if (__glibc_unlikely (workstart != NULL))
-	free (workstart);
-      workstart = NULL;
-
       /* Write the following constant string.  */
       outstring (specs[nspecs_done].end_of_fmt,
 		 specs[nspecs_done].next_fmt
 		 - specs[nspecs_done].end_of_fmt);
     }
  all_done:
-  if (__glibc_unlikely (workstart != NULL))
-    free (workstart);
   scratch_buffer_free (&argsbuf);
   scratch_buffer_free (&specsbuf);
   return done;