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annotate libtommath/bn_s_mp_sub.c @ 1930:299f4f19ba19

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Add /usr/sbin and /sbin to default root PATH When dropbear is used in a very restricted environment (such as in a initrd), the default user shell is often also very restricted and doesn't take care of setting the PATH so the user ends up with the PATH set by dropbear. Unfortunately, dropbear always sets "/usr/bin:/bin" as default PATH even for the root user which should have /usr/sbin and /sbin too. For a concrete instance of this problem, see the "Remote Unlocking" section in this tutorial: https://paxswill.com/blog/2013/11/04/encrypted-raspberry-pi/ It speaks of a bug in the initramfs script because it's written "blkid" instead of "/sbin/blkid"... this is just because the scripts from the initramfs do not expect to have a PATH without the sbin directories and because dropbear is not setting the PATH appropriately for the root user. I'm thus suggesting to use the attached patch to fix this misbehaviour (I did not test it, but it's easy enough). It might seem anecdotic but multiple Kali users have been bitten by this. From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903403
author Raphael Hertzog <hertzog@debian.org>
date Mon, 09 Jul 2018 16:27:53 +0200
parents 1051e4eea25a
children
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1 #include "tommath_private.h"
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2 #ifdef BN_S_MP_SUB_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
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4 /* SPDX-License-Identifier: Unlicense */
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6 /* low level subtraction (assumes |a| > |b|), HAC pp.595 Algorithm 14.9 */
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7 mp_err s_mp_sub(const mp_int *a, const mp_int *b, mp_int *c)
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8 {
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9 int olduse, min, max;
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10 mp_err err;
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12 /* find sizes */
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13 min = b->used;
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14 max = a->used;
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16 /* init result */
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17 if (c->alloc < max) {
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18 if ((err = mp_grow(c, max)) != MP_OKAY) {
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19 return err;
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20 }
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21 }
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22 olduse = c->used;
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23 c->used = max;
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25 {
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26 mp_digit u, *tmpa, *tmpb, *tmpc;
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27 int i;
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29 /* alias for digit pointers */
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30 tmpa = a->dp;
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31 tmpb = b->dp;
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32 tmpc = c->dp;
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33
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34 /* set carry to zero */
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35 u = 0;
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36 for (i = 0; i < min; i++) {
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37 /* T[i] = A[i] - B[i] - U */
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38 *tmpc = (*tmpa++ - *tmpb++) - u;
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40 /* U = carry bit of T[i]
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41 * Note this saves performing an AND operation since
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42 * if a carry does occur it will propagate all the way to the
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43 * MSB. As a result a single shift is enough to get the carry
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44 */
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45 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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47 /* Clear carry from T[i] */
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48 *tmpc++ &= MP_MASK;
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49 }
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51 /* now copy higher words if any, e.g. if A has more digits than B */
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52 for (; i < max; i++) {
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53 /* T[i] = A[i] - U */
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54 *tmpc = *tmpa++ - u;
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56 /* U = carry bit of T[i] */
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57 u = *tmpc >> (MP_SIZEOF_BITS(mp_digit) - 1u);
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59 /* Clear carry from T[i] */
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60 *tmpc++ &= MP_MASK;
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61 }
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63 /* clear digits above used (since we may not have grown result above) */
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64 MP_ZERO_DIGITS(tmpc, olduse - c->used);
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65 }
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67 mp_clamp(c);
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68 return MP_OKAY;
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69 }
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71 #endif