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Use explicit_bzero(3) to clear passwords, if it is available #353

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Use explicit_bzero(3) to clear passwords, if it is available #353

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10 changes: 10 additions & 0 deletions meson.build
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,15 @@ crypt = cc.find_library('crypt', required: not libpam.found())
math = cc.find_library('m')
rt = cc.find_library('rt')

have_explicit_bzero = cc.has_function(
'explicit_bzero',
args: ['-D_BSD_SOURCE'],
prefix: '#include <string.h>'
)
if not have_explicit_bzero
warning('Your system does not support explicit_bzero(3), using precarious fallback function to clear passwords')
endif

git = find_program('git', required: false)
scdoc = find_program('scdoc', required: get_option('man-pages'))
wayland_scanner_prog = find_program(wayland_scanner.get_variable('wayland_scanner'), native: true)
Expand Down Expand Up @@ -81,6 +90,7 @@ conf_data = configuration_data()
conf_data.set_quoted('SYSCONFDIR', get_option('prefix') / get_option('sysconfdir'))
conf_data.set_quoted('SWAYLOCK_VERSION', version)
conf_data.set10('HAVE_GDK_PIXBUF', gdk_pixbuf.found())
conf_data.set10('HAVE_EXPLICIT_BZERO', have_explicit_bzero)

subdir('include')

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5 changes: 5 additions & 0 deletions password.c
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
#define _BSD_SOURCE // for explicit_bzero
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@nmeum nmeum Mar 30, 2024 •
edited
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Defining feature tests macro in the source file is somewhat unideal, only doing it here because that's what is also done elsewhere in the code. Refer to this post for more information. Also using _BSD_SOURCE instead of _GNU_SOURCE here as the BSDs may not define the prototype otherwise. musl defines it on both _GNU_SOURCE and _BSD_SOURCE, not sure about glibc.

#include <assert.h>
#include <errno.h>
#include <pwd.h>
Expand All @@ -13,12 +14,16 @@
#include "unicode.h"

void clear_buffer(char *buf, size_t size) {
#ifdef HAVE_EXPLICIT_BZERO
explicit_bzero(buf, size);
#else
// Use volatile keyword so so compiler can't optimize this out.
volatile char *buffer = buf;
volatile char zero = '\0';
for (size_t i = 0; i < size; ++i) {
buffer[i] = zero;
}
#endif
}

void clear_password_buffer(struct swaylock_password *pw) {
Expand Down