Bug 20181 - open_memstream(): writes not at end of stream corrupt data
Summary: open_memstream(): writes not at end of stream corrupt data
Status: RESOLVED FIXED
Alias: None
Product: glibc
Classification: Unclassified
Component: stdio (show other bugs)
Version: unspecified
: P2 normal
Target Milestone: 2.25
Assignee: Adhemerval Zanella
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2016-05-30 19:05 IST by Andrew Church
Modified: 2016-09-30 16:50 IST (History)
2 users (show)

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Last reconfirmed:
fweimer: security-


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Description Andrew Church 2016-05-30 19:05:26 IST
Memory buffers opened with open_memstream() (and open_wmemstream()) corrupt one byte of data on flush after write if the current write position is not at the end of the stream.

The bug is in _IO_[w]mem_sync(), which blindly writes a null byte (or wchar) to the buffer whether or not the current write position is at the end of the buffer.  This appears to contravene POSIX, which says: "If a write moves the position to a value larger than the current length, the current length shall be set to this position. *In this case* a null character for open_memstream() or a null wide character for open_wmemstream() shall be appended to the current buffer." (emphasis added)

The fix is to remove the "else *fp->_IO_write_ptr = '\0'" clause (libio/memstream.c:115-116 and libio/wmemstream.c:115-116 in current git).

Simple testcase:

char *buf;
size_t size;
FILE *f;
assert(f = open_memstream(&buf, &size));
assert(fwrite("abc", 1, 3, f) == 3);
assert(fseek(f, 0, SEEK_SET) == 0);
assert(fwrite("z", 1, 1, f) == 1);
assert(fflush(f) == 0);  // Clobbers the "b".
assert(fseek(f, 3, SEEK_SET) == 0);  // Avoid truncating the buffer on close.
assert(fclose(f) == 0);
assert(size == 3);
assert(buf[0] == 'z');
assert(buf[1] == 'b');  // Fails (clobbered by null byte).
assert(buf[2] == 'c');
Comment 1 cvs-commit@gcc.gnu.org 2016-09-30 16:15:06 IST
This is an automated email from the git hooks/post-receive script. It was
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the project "GNU C Library master sources".

The branch, master has been updated
       via  645f97ced4d4b35deda3f8bde0927f898b163f5d (commit)
      from  f280fa6d171c4d3414c005ad2a7529e0d1d9ee0c (commit)

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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=645f97ced4d4b35deda3f8bde0927f898b163f5d

commit 645f97ced4d4b35deda3f8bde0927f898b163f5d
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Mon Jul 25 14:54:29 2016 -0300

    libio: Multiple fixes for open_{w}memstram (BZ#18241 and BZ#20181)
    
    This patches fixes multiples issues on open_{w}memstream reported on both
    BZ#18241 and BZ#20181:
    
      - failed fseek does not set errno.
      - negative offset in fseek fails even when resulting position is
        a valid one.
      - a flush after write if the current write position is not at the
        end of the stream currupt data.
    
    The main fix is on seek operation for memstream (_IO_{w}str_seekoff), where
    both _IO_read_ptr and _IO_read_end pointer are updated if a write operation
    has occured (similar to default file operations).  Also, to calculate the
    offset on both read and write pointers, a temporary value is instead of
    updating the argument supplied value.  Negative offset are valid if resulting
    internal pointer is within the range of _IO_{read,write}_base and
    _IO_{read,write}_end.
    
    Also POSIX states that a null or wide null shall be appended to the current
    buffer iff a write moves the position to a value larger than the current
    lenght.  Current implementation appends a null or wide null regardless
    of this condition.  This patch fixes it by removing the 'else' condition
    on _IO_{w}mem_sync.
    
    Checked on x86_64.
    
    	[BZ #18241]
    	[BZ #20181]
    	* libio/Makefile (test): Add tst-memstream3 and tst-wmemstream3.
    	* libio/memstream.c (_IO_mem_sync): Only append a null byte if
    	write position is at the end the buffer.
    	* libio/wmemstream.c (_IO_wmem_sync): Likewise.
    	* libio/strops.c (_IO_str_switch_to_get_mode): New function.
    	(_IO_str_seekoff): Set correct offset from negative displacement and
    	set EINVAL for invalid ones.
    	* libio/wstrops.c (enlarge_userbuf): Use correct function to calculate
    	buffer length.
    	(_IO_wstr_switch_to_get_mode): New function.
    	(_IO_wstr_seekoff): Set correct offset from negative displacement and
    	set EINVAL for invalid ones.
    	* libio/tst-memstream3.c: New file.
    	* libio/tst-wmemstream3.c: Likewise.
    	* manual/examples/memstrm.c: Remove warning when priting size_t.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                                          |   20 +++
 libio/Makefile                                     |    4 +-
 libio/memstream.c                                  |    2 -
 libio/strops.c                                     |   81 +++++++---
 libio/tst-memstream3.c                             |  165 ++++++++++++++++++++
 .../tst-wcstod-round.c => libio/tst-wmemstream3.c  |   33 +++--
 libio/wmemstream.c                                 |    2 -
 libio/wstrops.c                                    |   89 +++++++----
 manual/examples/memstrm.c                          |    4 +-
 9 files changed, 327 insertions(+), 73 deletions(-)
 create mode 100644 libio/tst-memstream3.c
 copy wcsmbs/tst-wcstod-round.c => libio/tst-wmemstream3.c (55%)
Comment 2 Adhemerval Zanella 2016-09-30 16:50:35 IST
Fixed by 645f97ced4d4b35deda3f8bde0927f898b163f5d.