Bug 26649 (CVE-2020-29573) - printf should handle non-normal x86 long double numbers gracefully (CVE-2020-29573)
Summary: printf should handle non-normal x86 long double numbers gracefully (CVE-2020-...
Status: RESOLVED FIXED
Alias: CVE-2020-29573
Product: glibc
Classification: Unclassified
Component: stdio (show other bugs)
Version: unspecified
: P2 normal
Target Milestone: 2.33
Assignee: Florian Weimer
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2020-09-22 14:24 UTC by Florian Weimer
Modified: 2020-12-16 10:27 UTC (History)
3 users (show)

See Also:
Host:
Target:
Build:
Last reconfirmed:
siddhesh: security+


Attachments

Note You need to log in before you can comment on or make changes to this bug.
Description Florian Weimer 2020-09-22 14:24:34 UTC
The behavior of isnan/__builtin_isnan on bit patterns that do not correspond to something that the CPU would produce from valid inputs is currently under-defined in the toolchain. (The GCC built-in and glibc disagree.)

The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c assumes the GCC behavior that returns true for non-normal numbers which are not specified as NaN. (The glibc implementation returns false for such numbers.)

At present, passing non-normal numbers to __mppn_extract_long_double causes this function to produce irregularly shaped multi-precision integers, triggering undefined behavior in __printf_fp_l.

With GCC 10 and glibc 2.32, this behavior is not visible because __builtin_isnan is used, which avoids calling __mppn_extract_long_double in this case.
Comment 1 Florian Weimer 2020-09-22 14:32:44 UTC
Patch posted: https://sourceware.org/pipermail/libc-alpha/2020-September/117779.html
Comment 2 Florian Weimer 2020-09-22 17:15:29 UTC
Fixed for glibc 2.33 via:

commit 681900d29683722b1cb0a8e565a0585846ec5a61
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Sep 22 19:07:48 2020 +0200

    x86: Harden printf against non-normal long double values (bug 26649)
    
    The behavior of isnan/__builtin_isnan on bit patterns that do not
    correspond to something that the CPU would produce from valid inputs
    is currently under-defined in the toolchain. (The GCC built-in and
    glibc disagree.)
    
    The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c
    assumes the GCC behavior that returns true for non-normal numbers
    which are not specified as NaN. (The glibc implementation returns
    false for such numbers.)
    
    At present, passing non-normal numbers to __mpn_extract_long_double
    causes this function to produce irregularly shaped multi-precision
    integers, triggering undefined behavior in __printf_fp_l.
    
    With GCC 10 and glibc 2.32, this behavior is not visible because
    __builtin_isnan is used, which avoids calling
    __mpn_extract_long_double in this case.  This commit updates the
    implementation of __mpn_extract_long_double so that regularly shaped
    multi-precision integers are produced in this case, avoiding
    undefined behavior in __printf_fp_l.