memory leak in mpihelp_mul_karatsuba_case

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    abf02e29 Merge tag 'pm-5.2-rc6' of git://git.kernel.org/pu..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17a8bfeaa00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=56f1da14935c3cce
dashboard link: https://syzkaller.appspot.com/bug?extid=f7baccc38dcc1e094e77
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=171aa7e6a00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=153306cea00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f7baccc38dcc1e094e77@xxxxxxxxxxxxxxxxxxxxxxxxx

ffffffffda RBX: 0000000000000000 RCX: 0000000000441ac9
BUG: memory leak
unreferenced object 0xffff88811f4da200 (size 512):
  comm "syz-executor301", pid 7045, jiffies 4294955450 (age 7.850s)
  hex dump (first 32 bytes):
    ad dc f4 43 66 b0 1a 88 8f 0c 17 d5 86 34 3a 85  ...Cf........4:.
    e3 63 c8 bf 2e 3b f5 0d 1c ab 63 30 15 fe a1 e9  .c...;....c0....
  backtrace:
    [<00000000d5589961>] kmemleak_alloc_recursive  
include/linux/kmemleak.h:43 [inline]
    [<00000000d5589961>] slab_post_alloc_hook mm/slab.h:439 [inline]
    [<00000000d5589961>] slab_alloc mm/slab.c:3326 [inline]
    [<00000000d5589961>] __do_kmalloc mm/slab.c:3658 [inline]
    [<00000000d5589961>] __kmalloc+0x161/0x2c0 mm/slab.c:3669
    [<00000000022eaa00>] kmalloc include/linux/slab.h:552 [inline]
    [<00000000022eaa00>] mpi_alloc_limb_space+0x29/0x50 lib/mpi/mpiutil.c:64
    [<00000000d637c699>] mpihelp_mul_karatsuba_case+0x67/0x460  
lib/mpi/mpih-mul.c:331
    [<00000000401dc6f9>] mpi_powm+0x7b0/0xdd0 lib/mpi/mpi-pow.c:225
    [<00000000be8dcb84>] _compute_val crypto/dh.c:39 [inline]
    [<00000000be8dcb84>] dh_compute_value+0x160/0x220 crypto/dh.c:178
    [<00000000471846ad>] crypto_kpp_generate_public_key  
include/crypto/kpp.h:315 [inline]
    [<00000000471846ad>] __keyctl_dh_compute+0x447/0x970  
security/keys/dh.c:367
    [<000000002f6d650d>] keyctl_dh_compute+0x67/0xa6 security/keys/dh.c:422
    [<00000000b798bc7f>] __do_sys_keyctl security/keys/keyctl.c:1737  
[inline]
    [<00000000b798bc7f>] __se_sys_keyctl security/keys/keyctl.c:1633  
[inline]
    [<00000000b798bc7f>] __x64_sys_keyctl+0xa5/0x330  
security/keys/keyctl.c:1633
    [<000000007a6f9515>] do_syscall_64+0x76/0x1a0  
arch/x86/entry/common.c:301
    [<00000000057f2768>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

BUG: memory leak
unreferenced object 0xffff88811f4dac00 (size 512):
  comm "syz-executor301", pid 7045, jiffies 4294955450 (age 7.850s)
  hex dump (first 32 bytes):
    62 72 c4 ae ac af a3 ba e5 24 da a5 30 5e cb c4  br.......$..0^..
    a6 46 44 39 76 2e 42 f6 85 6a 5b ad ae 97 4e 83  .FD9v.B..j[...N.
  backtrace:
    [<00000000d5589961>] kmemleak_alloc_recursive  
include/linux/kmemleak.h:43 [inline]
    [<00000000d5589961>] slab_post_alloc_hook mm/slab.h:439 [inline]
    [<00000000d5589961>] slab_alloc mm/slab.c:3326 [inline]
    [<00000000d5589961>] __do_kmalloc mm/slab.c:3658 [inline]
    [<00000000d5589961>] __kmalloc+0x161/0x2c0 mm/slab.c:3669
    [<00000000022eaa00>] kmalloc include/linux/slab.h:552 [inline]
    [<00000000022eaa00>] mpi_alloc_limb_space+0x29/0x50 lib/mpi/mpiutil.c:64
    [<0000000025804541>] mpihelp_mul_karatsuba_case+0x394/0x460  
lib/mpi/mpih-mul.c:346
    [<00000000401dc6f9>] mpi_powm+0x7b0/0xdd0 lib/mpi/mpi-pow.c:225
    [<00000000be8dcb84>] _compute_val crypto/dh.c:39 [inline]
    [<00000000be8dcb84>] dh_compute_value+0x160/0x220 crypto/dh.c:178
    [<00000000471846ad>] crypto_kpp_generate_public_key  
include/crypto/kpp.h:315 [inline]
    [<00000000471846ad>] __keyctl_dh_compute+0x447/0x970  
security/keys/dh.c:367
    [<000000002f6d650d>] keyctl_dh_compute+0x67/0xa6 security/keys/dh.c:422
    [<00000000b798bc7f>] __do_sys_keyctl security/keys/keyctl.c:1737  
[inline]
    [<00000000b798bc7f>] __se_sys_keyctl security/keys/keyctl.c:1633  
[inline]
    [<00000000b798bc7f>] __x64_sys_keyctl+0xa5/0x330  
security/keys/keyctl.c:1633
    [<000000007a6f9515>] do_syscall_64+0x76/0x1a0  
arch/x86/entry/common.c:301
    [<00000000057f2768>] entry_SYSCALL_64_after_hwframe+0x44/0xa9



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