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fc5f9cc46d
Writing into allocated memory right before calling free can be optimized
away by smart compilers. To prevent this, a volatile access must be
performed. This happens already in crypt_safe_memzero.
It was difficult to provoke GCC to remove the assignment, but I was able
to find a way to prove the theory:
* Build cryptsetup with: CFLAGS="-flto -O3 -g" ./configure --enable-static
* Create main.c:
#include <libcryptsetup.h>
int
main(void) {
char *x = crypt_safe_alloc(64);
crypt_safe_free(x);
return 0;
}
* Build the program with: gcc -O3 -flto -static -o main main.c -lcryptsetup
* Disassemble: objdump -d main
My output on an amd64 system is:
0000000000401670 <main>:
401670: 41 54 push %r12
401672: bf f0 03 00 00 mov $0x3f0,%edi
401677: 55 push %rbp
401678: 48 83 ec 08 sub $0x8,%rsp
40167c: e8 ff 4d 01 00 callq 416480 <__libc_malloc>
401681: 48 85 c0 test %rax,%rax
401684: 74 2f je 4016b5 <main+0x45>
401686: 48 c7 00 e8 03 00 00 movq $0x3e8,(%rax)
40168d: 4c 8d 60 08 lea 0x8(%rax),%r12
401691: 48 89 c5 mov %rax,%rbp
401694: be e8 03 00 00 mov $0x3e8,%esi
401699: 4c 89 e7 mov %r12,%rdi
40169c: e8 4f 76 01 00 callq 418cf0 <explicit_bzero>
4016a1: 48 8b 75 00 mov 0x0(%rbp),%rsi
4016a5: 4c 89 e7 mov %r12,%rdi
4016a8: e8 43 76 01 00 callq 418cf0 <explicit_bzero>
4016ad: 48 89 ef mov %rbp,%rdi
4016b0: e8 3b 54 01 00 callq 416af0 <__free>
4016b5: 48 83 c4 08 add $0x8,%rsp
4016b9: 31 c0 xor %eax,%eax
4016bb: 5d pop %rbp
4016bc: 41 5c pop %r12
4016be: c3 retq
4016bf: 90 nop
You can see that the memory allocation and explicit_bzero calls were not
optimized away. But the size assignment disappeared.
Compiling without -O3 or without -flto does not inline the calls and
keeps the assignment. Also the shared library shipped with my
distribution has the assignment.
105 lines
2.4 KiB
C
105 lines
2.4 KiB
C
/*
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* utils_safe_memory - safe memory helpers
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*
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* Copyright (C) 2009-2020 Red Hat, Inc. All rights reserved.
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* Copyright (C) 2009-2020 Milan Broz
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version 2
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* of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
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*/
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#include <stdlib.h>
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#include <string.h>
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#include "libcryptsetup.h"
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struct safe_allocation {
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size_t size;
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char data[0];
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};
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/*
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* Replacement for memset(s, 0, n) on stack that can be optimized out
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* Also used in safe allocations for explicit memory wipe.
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*/
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void crypt_safe_memzero(void *data, size_t size)
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{
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#ifdef HAVE_EXPLICIT_BZERO
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explicit_bzero(data, size);
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#else
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volatile uint8_t *p = (volatile uint8_t *)data;
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while(size--)
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*p++ = 0;
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#endif
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}
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/* safe allocations */
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void *crypt_safe_alloc(size_t size)
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{
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struct safe_allocation *alloc;
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if (!size || size > (SIZE_MAX - offsetof(struct safe_allocation, data)))
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return NULL;
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alloc = malloc(size + offsetof(struct safe_allocation, data));
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if (!alloc)
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return NULL;
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alloc->size = size;
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crypt_safe_memzero(&alloc->data, size);
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/* coverity[leaked_storage] */
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return &alloc->data;
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}
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void crypt_safe_free(void *data)
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{
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struct safe_allocation *alloc;
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volatile size_t *s;
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if (!data)
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return;
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alloc = (struct safe_allocation *)
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((char *)data - offsetof(struct safe_allocation, data));
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crypt_safe_memzero(data, alloc->size);
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s = (volatile size_t *)&alloc->size;
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*s = 0x55aa55aa;
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free(alloc);
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}
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void *crypt_safe_realloc(void *data, size_t size)
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{
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struct safe_allocation *alloc;
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void *new_data;
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new_data = crypt_safe_alloc(size);
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if (new_data && data) {
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alloc = (struct safe_allocation *)
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((char *)data - offsetof(struct safe_allocation, data));
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if (size > alloc->size)
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size = alloc->size;
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memcpy(new_data, data, size);
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}
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crypt_safe_free(data);
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return new_data;
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}
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