Comment # 1 on bug 1188327 from Mian Yousaf Kaukab

ecdsa_set_pub_key() is making a u64 pointer at 1 byte offset of the key. 

const unsigned char *d = key;
const u64 *digits = (const u64 *)&d[1];

So digits will mostly be unaligned.

ecc_swap_digits() is then using ldm instruction on this unaligned pointer which
is causing the exception. 

There are two instances of ecc_swap_digits(). First instance is implemented by
two ldr instructions. As ldr can handle unaligned addresses, it works.

The second instance is implemented by an ldm instruction which can't handle
unaligned addresses and causes the exception.

Here is the disassembly of the code:
/* First instance */
../crypto/ecc.h:
54        for (i = 0; i < ndigits; i++)
   0xc07c26f0 <+132>:    cmp    r3, #0
   0xc07c26f4 <+136>:    beq    0xc07c276c <ecdsa_set_pub_key+256>
   0xc07c26f8 <+140>:    add    r1, r5, #1
   0xc07c26fc <+144>:    sub    r6, r3, #1
   0xc07c2700 <+148>:    lsl    r5, r3, #3
   0xc07c2704 <+152>:    add    r12, r4, #72    ; 0x48
   0xc07c2708 <+156>:    add    r3, r1, r3, lsl #3

55            out[i] = be64_to_cpu(src[ndigits - 1 - i]);
   0xc07c270c <+160>:    ldr    r2, [r3, #-8]! //<== First load ==
   0xc07c2710 <+164>:    ldr    r0, [r3, #4]   //<== Second load ==
   0xc07c2714 <+168>:    rev    r2, r2
   0xc07c2718 <+172>:    rev    r0, r0
   0xc07c271c <+176>:    str    r0, [r12, #8]!
   0xc07c2720 <+180>:    cmp    r3, r1
   0xc07c2724 <+184>:    str    r2, [r12, #4]

/* Second instance */

../crypto/ecdsa.c:
246        ecc_swap_digits(&digits[ndigits], ctx->pub_key.y, ndigits);
   0xc07c272c <+192>:    ldr    r0, [r4, #212]    ; 0xd4

../crypto/ecc.h:
54        for (i = 0; i < ndigits; i++)
   0xc07c2730 <+196>:    bic    lr, lr, #7
   0xc07c2734 <+200>:    add    lr, lr, r6, lsl #3
   0xc07c2738 <+204>:    sub    r5, r5, #8
   0xc07c273c <+208>:    sub    r2, r0, #8
   0xc07c2740 <+212>:    add    r3, r3, lr
   0xc07c2744 <+216>:    add    r0, r0, r5

55            out[i] = be64_to_cpu(src[ndigits - 1 - i]);
   0xc07c2748 <+220>:    ldm    r3, {r1, r12} //<== ldm thus alignment
exception
   0xc07c274c <+224>:    rev    r12, r12
   0xc07c2750 <+228>:    str    r12, [r2, #8]!
   0xc07c2754 <+232>:    rev    r1, r1
   0xc07c2758 <+236>:    cmp    r0, r2
   0xc07c275c <+240>:    sub    r3, r3, #8
   0xc07c2760 <+244>:    str    r1, [r2, #4]