man3/bzero.3

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   2 .\" Copyright (C) 2017 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH bzero 3 (date) "Linux man-pages (unreleased)"
   7 .SH NAME
   8 bzero, explicit_bzero \- zero a byte string
   9 .SH LIBRARY
  10 Standard C library
  11 .RI ( libc ", " \-lc )
  12 .SH SYNOPSIS
  13 .nf
  14 .B #include <strings.h>
  15 .PP
  16 .BI "void bzero(void " s [. n "], size_t " n );
  17 .PP
  18 .B #include <string.h>
  19 .PP
  20 .BI "void explicit_bzero(void " s [. n "], size_t " n );
  21 .fi
  22 .SH DESCRIPTION
  23 The
  24 .BR bzero ()
  25 function erases the data in the
  26 .I n
  27 bytes of the memory starting at the location pointed to by
  28 .IR s ,
  29 by writing zeros (bytes containing \[aq]\e0\[aq]) to that area.
  30 .PP
  31 The
  32 .BR explicit_bzero ()
  33 function performs the same task as
  34 .BR bzero ().
  35 It differs from
  36 .BR bzero ()
  37 in that it guarantees that compiler optimizations will not remove the
  38 erase operation if the compiler deduces that the operation is "unnecessary".
  39 .SH RETURN VALUE
  40 None.
  41 .SH ATTRIBUTES
  42 For an explanation of the terms used in this section, see
  43 .BR attributes (7).
  44 .TS
  45 allbox;
  46 lbx lb lb
  47 l l l.
  48 Interface       Attribute       Value
  49 T{
  50 .na
  51 .nh
  52 .BR bzero (),
  53 .BR explicit_bzero ()
  54 T}      Thread safety   MT-Safe
  55 .TE
  56 .sp 1
  57 .SH STANDARDS
  58 None.
  59 .SH HISTORY
  60 .TP
  61 .BR explicit_bzero ()
  62 glibc 2.25.
  63 .IP
  64 The
  65 .BR explicit_bzero ()
  66 function is a nonstandard extension that is also present on some of the BSDs.
  67 Some other implementations have a similar function, such as
  68 .BR memset_explicit ()
  69 or
  70 .BR memset_s ().
  71 .TP
  72 .BR bzero ()
  73 4.3BSD.
  74 .IP
  75 Marked as LEGACY in POSIX.1-2001.
  76 Removed in POSIX.1-2008.
  77 .SH NOTES
  78 The
  79 .BR explicit_bzero ()
  80 function addresses a problem that security-conscious applications
  81 may run into when using
  82 .BR bzero ():
  83 if the compiler can deduce that the location to be zeroed will
  84 never again be touched by a
  85 .I correct
  86 program, then it may remove the
  87 .BR bzero ()
  88 call altogether.
  89 This is a problem if the intent of the
  90 .BR bzero ()
  91 call was to erase sensitive data (e.g., passwords)
  92 to prevent the possibility that the data was leaked
  93 by an incorrect or compromised program.
  94 Calls to
  95 .BR explicit_bzero ()
  96 are never optimized away by the compiler.
  97 .PP
  98 The
  99 .BR explicit_bzero ()
 100 function does not solve all problems associated with erasing sensitive data:
 101 .IP \[bu] 3
 102 The
 103 .BR explicit_bzero ()
 104 function does
 105 .I not
 106 guarantee that sensitive data is completely erased from memory.
 107 (The same is true of
 108 .BR bzero ().)
 109 For example, there may be copies of the sensitive data in
 110 a register and in "scratch" stack areas.
 111 The
 112 .BR explicit_bzero ()
 113 function is not aware of these copies, and can't erase them.
 114 .IP \[bu]
 115 In some circumstances,
 116 .BR explicit_bzero ()
 117 can
 118 .I decrease
 119 security.
 120 If the compiler determined that the variable containing the
 121 sensitive data could be optimized to be stored in a register
 122 (because it is small enough to fit in a register,
 123 and no operation other than the
 124 .BR explicit_bzero ()
 125 call would need to take the address of the variable), then the
 126 .BR explicit_bzero ()
 127 call will force the data to be copied from the register
 128 to a location in RAM that is then immediately erased
 129 (while the copy in the register remains unaffected).
 130 The problem here is that data in RAM is more likely to be exposed
 131 by a bug than data in a register, and thus the
 132 .BR explicit_bzero ()
 133 call creates a brief time window where the sensitive data is more
 134 vulnerable than it would otherwise have been
 135 if no attempt had been made to erase the data.
 136 .PP
 137 Note that declaring the sensitive variable with the
 138 .B volatile
 139 qualifier does
 140 .I not
 141 eliminate the above problems.
 142 Indeed, it will make them worse, since, for example,
 143 it may force a variable that would otherwise have been optimized
 144 into a register to instead be maintained in (more vulnerable)
 145 RAM for its entire lifetime.
 146 .PP
 147 Notwithstanding the above details, for security-conscious applications, using
 148 .BR explicit_bzero ()
 149 is generally preferable to not using it.
 150 The developers of
 151 .BR explicit_bzero ()
 152 anticipate that future compilers will recognize calls to
 153 .BR explicit_bzero ()
 154 and take steps to ensure that all copies of the sensitive data are erased,
 155 including copies in registers or in "scratch" stack areas.
 156 .SH SEE ALSO
 157 .BR bstring (3),
 158 .BR memset (3),
 159 .BR swab (3)