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Subjectfs,seq_file: huge allocations for seq file reads
Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel, I've stumbled on the following:

[ 2052.444910] WARNING: CPU: 3 PID: 26525 at mm/page_alloc.c:2513 __alloc_pages_slowpat
h+0x6a/0x801()
[ 2052.447575] Modules linked in:
[ 2052.448438] CPU: 3 PID: 26525 Comm: trinity-c3 Tainted: G W 3.14.0-next-2
0140407-sasha-00023-gd35b0d6 #382
[ 2052.452147] 0000000000000009 ffff88010f485af8 ffffffff9d52ee51 0000000000005d20
[ 2052.454425] 0000000000000000 ffff88010f485b38 ffffffff9a15a2dc 000000174802a016
[ 2052.456676] 00000000001040d0 0000000000000000 0000000000000002 0000000000000000
[ 2052.458851] Call Trace:
[ 2052.459587] dump_stack (lib/dump_stack.c:52)
[ 2052.461211] warn_slowpath_common (kernel/panic.c:419)
[ 2052.462914] warn_slowpath_null (kernel/panic.c:454)
[ 2052.464512] __alloc_pages_slowpath (mm/page_alloc.c:2513 (discriminator 3))
[ 2052.466160] ? get_page_from_freelist (mm/page_alloc.c:1939)
[ 2052.468020] ? sched_clock_local (kernel/sched/clock.c:213)
[ 2052.469633] ? get_parent_ip (kernel/sched/core.c:2471)
[ 2052.471329] __alloc_pages_nodemask (mm/page_alloc.c:2766)
[ 2052.473084] alloc_pages_current (mm/mempolicy.c:2131)
[ 2052.474688] ? __get_free_pages (mm/page_alloc.c:2803)
[ 2052.476273] ? __free_pages_ok (arch/x86/include/asm/paravirt.h:809 (discriminator 2) mm/page_alloc.c:766 (discriminator 2))
[ 2052.477980] __get_free_pages (mm/page_alloc.c:2803)
[ 2052.481082] kmalloc_order_trace (include/linux/slab.h:379 mm/slab_common.c:525)
[ 2052.483193] __kmalloc (include/linux/slab.h:396 mm/slub.c:3303)
[ 2052.485417] ? kfree (mm/slub.c:3395)
[ 2052.486337] traverse (fs/seq_file.c:141)
[ 2052.487289] seq_read (fs/seq_file.c:179 (discriminator 1))
[ 2052.488161] vfs_read (fs/read_write.c:408)
[ 2052.489001] SyS_pread64 (include/linux/file.h:38 fs/read_write.c:557 fs/read_write.c:544)
[ 2052.489959] tracesys (arch/x86/kernel/entry_64.S:749)

It seems that when we attempt to read huge chunks of data from a seq file
there would be no check for the size being read, leading to the kernel
attempting to allocate huge chunks of data internally.

As far as I remember, there was a PAGE_SIZE limitation on those, but I'm
not certain about that. Could someone please confirm it?


Thanks,
Sasha


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