man3/bzero.3

   1 .\" Copyright (C) 2017 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .TH BZERO 3  2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   6 .SH NAME
   7 bzero, explicit_bzero \- zero a byte string
   8 .SH LIBRARY
   9 Standard C library
  10 .RI ( libc ", " \-lc )
  11 .SH SYNOPSIS
  12 .nf
  13 .B #include <strings.h>
  14 .PP
  15 .BI "void bzero(void *" s ", size_t " n );
  16 .PP
  17 .B #include <string.h>
  18 .PP
  19 .BI "void explicit_bzero(void *" s ", size_t " n );
  20 .fi
  21 .SH DESCRIPTION
  22 The
  23 .BR bzero ()
  24 function erases the data in the
  25 .I n
  26 bytes of the memory starting at the location pointed to by
  27 .IR s ,
  28 by writing zeros (bytes containing \(aq\e0\(aq) to that area.
  29 .PP
  30 The
  31 .BR explicit_bzero ()
  32 function performs the same task as
  33 .BR bzero ().
  34 It differs from
  35 .BR bzero ()
  36 in that it guarantees that compiler optimizations will not remove the
  37 erase operation if the compiler deduces that the operation is "unnecessary".
  38 .SH RETURN VALUE
  39 None.
  40 .SH VERSIONS
  41 .BR explicit_bzero ()
  42 first appeared in glibc 2.25.
  43 .SH ATTRIBUTES
  44 For an explanation of the terms used in this section, see
  45 .BR attributes (7).
  46 .ad l
  47 .nh
  48 .TS
  49 allbox;
  50 lbx lb lb
  51 l l l.
  52 Interface       Attribute       Value
  53 T{
  54 .BR bzero (),
  55 .BR explicit_bzero ()
  56 T}      Thread safety   MT-Safe
  57 .TE
  58 .hy
  59 .ad
  60 .sp 1
  61 .SH STANDARDS
  62 The
  63 .BR bzero ()
  64 function is deprecated (marked as LEGACY in POSIX.1-2001); use
  65 .BR memset (3)
  66 in new programs.
  67 POSIX.1-2008 removes the specification of
  68 .BR bzero ().
  69 The
  70 .BR bzero ()
  71 function first appeared in 4.3BSD.
  72 .PP
  73 The
  74 .BR explicit_bzero ()
  75 function is a nonstandard extension that is also present on some of the BSDs.
  76 Some other implementations have a similar function, such as
  77 .BR memset_explicit ()
  78 or
  79 .BR memset_s ().
  80 .SH NOTES
  81 The
  82 .BR explicit_bzero ()
  83 function addresses a problem that security-conscious applications
  84 may run into when using
  85 .BR bzero ():
  86 if the compiler can deduce that the location to be zeroed will
  87 never again be touched by a
  88 .I correct
  89 program, then it may remove the
  90 .BR bzero ()
  91 call altogether.
  92 This is a problem if the intent of the
  93 .BR bzero ()
  94 call was to erase sensitive data (e.g., passwords)
  95 to prevent the possibility that the data was leaked
  96 by an incorrect or compromised program.
  97 Calls to
  98 .BR explicit_bzero ()
  99 are never optimized away by the compiler.
 100 .PP
 101 The
 102 .BR explicit_bzero ()
 103 function does not solve all problems associated with erasing sensitive data:
 104 .IP 1. 3
 105 The
 106 .BR explicit_bzero ()
 107 function does
 108 .I not
 109 guarantee that sensitive data is completely erased from memory.
 110 (The same is true of
 111 .BR bzero ().)
 112 For example, there may be copies of the sensitive data in
 113 a register and in "scratch" stack areas.
 114 The
 115 .BR explicit_bzero ()
 116 function is not aware of these copies, and can't erase them.
 117 .IP 2.
 118 In some circumstances,
 119 .BR explicit_bzero ()
 120 can
 121 .I decrease
 122 security.
 123 If the compiler determined that the variable containing the
 124 sensitive data could be optimized to be stored in a register
 125 (because it is small enough to fit in a register,
 126 and no operation other than the
 127 .BR explicit_bzero ()
 128 call would need to take the address of the variable), then the
 129 .BR explicit_bzero ()
 130 call will force the data to be copied from the register
 131 to a location in RAM that is then immediately erased
 132 (while the copy in the register remains unaffected).
 133 The problem here is that data in RAM is more likely to be exposed
 134 by a bug than data in a register, and thus the
 135 .BR explicit_bzero ()
 136 call creates a brief time window where the sensitive data is more
 137 vulnerable than it would otherwise have been
 138 if no attempt had been made to erase the data.
 139 .PP
 140 Note that declaring the sensitive variable with the
 141 .B volatile
 142 qualifier does
 143 .I not
 144 eliminate the above problems.
 145 Indeed, it will make them worse, since, for example,
 146 it may force a variable that would otherwise have been optimized
 147 into a register to instead be maintained in (more vulnerable)
 148 RAM for its entire lifetime.
 149 .PP
 150 Notwithstanding the above details, for security-conscious applications, using
 151 .BR explicit_bzero ()
 152 is generally preferable to not using it.
 153 The developers of
 154 .BR explicit_bzero ()
 155 anticipate that future compilers will recognize calls to
 156 .BR explicit_bzero ()
 157 and take steps to ensure that all copies of the sensitive data are erased,
 158 including copies in registers or in "scratch" stack areas.
 159 .SH SEE ALSO
 160 .BR bstring (3),
 161 .BR memset (3),
 162 .BR swab (3)