memory leak in bio_copy_user_iov

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    0011572c Merge branch 'for-5.2-fixes' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15193256a00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=cb38d33cd06d8d48
dashboard link: https://syzkaller.appspot.com/bug?extid=03e5c8ebd22cc6c3a8cb
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13244221a00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=117b2432a00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+03e5c8ebd22cc6c3a8cb@xxxxxxxxxxxxxxxxxxxxxxxxx

ram
executing program
BUG: memory leak
unreferenced object 0xffff8881204d7800 (size 2048):
  comm "syz-executor855", pid 6936, jiffies 4294941958 (age 26.780s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    20 00 00 00 02 01 00 00 00 00 00 00 08 00 00 00   ...............
  backtrace:
    [<00000000c5e27070>] kmemleak_alloc_recursive  
include/linux/kmemleak.h:43 [inline]
    [<00000000c5e27070>] slab_post_alloc_hook mm/slab.h:439 [inline]
    [<00000000c5e27070>] slab_alloc mm/slab.c:3326 [inline]
    [<00000000c5e27070>] __do_kmalloc mm/slab.c:3658 [inline]
    [<00000000c5e27070>] __kmalloc+0x161/0x2c0 mm/slab.c:3669
    [<000000004415e750>] kmalloc include/linux/slab.h:552 [inline]
    [<000000004415e750>] bio_alloc_bioset+0x1b8/0x2c0 block/bio.c:439
    [<000000002da58d1d>] bio_kmalloc include/linux/bio.h:391 [inline]
    [<000000002da58d1d>] bio_copy_user_iov+0x113/0x4a0 block/bio.c:1275
    [<00000000b4b23d95>] __blk_rq_map_user_iov block/blk-map.c:67 [inline]
    [<00000000b4b23d95>] blk_rq_map_user_iov+0xc6/0x2b0 block/blk-map.c:136
    [<00000000edad5f7e>] blk_rq_map_user+0x71/0xb0 block/blk-map.c:166
    [<00000000c94723b5>] sg_start_req drivers/scsi/sg.c:1813 [inline]
    [<00000000c94723b5>] sg_common_write.isra.0+0x619/0xa10  
drivers/scsi/sg.c:809
    [<00000000b11f3605>] sg_write.part.0+0x325/0x570 drivers/scsi/sg.c:709
    [<00000000aba41953>] sg_write+0x44/0x64 drivers/scsi/sg.c:617
    [<00000000afecd177>] __vfs_write+0x43/0xa0 fs/read_write.c:494
    [<00000000de690898>] vfs_write fs/read_write.c:558 [inline]
    [<00000000de690898>] vfs_write+0xee/0x210 fs/read_write.c:542
    [<00000000705a35b0>] ksys_write+0x7c/0x130 fs/read_write.c:611
    [<000000009efb9e6c>] __do_sys_write fs/read_write.c:623 [inline]
    [<000000009efb9e6c>] __se_sys_write fs/read_write.c:620 [inline]
    [<000000009efb9e6c>] __x64_sys_write+0x1e/0x30 fs/read_write.c:620
    [<00000000f9e48771>] do_syscall_64+0x76/0x1a0  
arch/x86/entry/common.c:301
    [<00000000d5cff9fc>] entry_SYSCALL_64_after_hwframe+0x44/0xa9



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