Bug 26649 (CVE-2020-29573)

Summary:	printf should handle non-normal x86 long double numbers gracefully (CVE-2020-29573)
Product:	glibc	Reporter:	Florian Weimer <fweimer>
Component:	stdio	Assignee:	Florian Weimer <fweimer>
Status:	RESOLVED FIXED
Severity:	normal	CC:	bruno, carnil, fweimer, siddhesh
Priority:	P2	Flags:	siddhesh: security+
Version:	unspecified
Target Milestone:	2.33
See Also:	https://sourceware.org/bugzilla/show_bug.cgi?id=4586 https://bugzilla.redhat.com/show_bug.cgi?id=1869380
Host:		Target:
Build:		Last reconfirmed:

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.

Comment 3 Siddhesh Poyarekar 2023-07-25 14:07:31 UTC

*** Bug 4586 has been marked as a duplicate of this bug. ***