@@ -50,6 +50,7 @@
#define PFX RNG_MODULE_NAME ": "
#define RNG_MISCDEV_MINOR 183 /* official */
+#define EARLY_RANDOMNESS_SIZE 16
static struct hwrng *current_rng;
static struct task_struct *hwrng_fill;
@@ -84,14 +85,37 @@ static size_t rng_buffer_size(void)
static void add_early_randomness(struct hwrng *rng)
{
- unsigned char bytes[16];
+ unsigned char *bytes;
int bytes_read;
+ /*
+ * This code can be reached with rng_mutex held, through the following
+ * call chain:
+ *
+ * hwrng_attr_current_store()
+ * set_current_rng()
+ * hwrng_init()
+ * add_early_randomness()
+ *
+ * (that is, when a different RNG is selected through the "rng_current"
+ * sysfs attribute). For that reason, allocate memory without enabling
+ * sleep.
+ *
+ * If the (immediate) allocation fails, we just pretend to have read
+ * zero bytes from the RNG, as that is already valid behavior. Also,
+ * feeding initial randomness from the device to the system entropy
+ * pool is not important enough to tap into emergency memory pools.
+ */
+ bytes = kmalloc(EARLY_RANDOMNESS_SIZE, GFP_NOWAIT | __GFP_NOWARN);
+ if (!bytes)
+ return;
+
mutex_lock(&reading_mutex);
- bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
+ bytes_read = rng_get_data(rng, bytes, EARLY_RANDOMNESS_SIZE, 1);
mutex_unlock(&reading_mutex);
if (bytes_read > 0)
add_device_randomness(bytes, bytes_read);
+ kfree(bytes);
}
static inline void cleanup_rng(struct kref *kref)
The virtio-rng backend for hwrng passes the buffer that it receives for filling to sg_set_buf() directly, in: virtio_read() [drivers/char/hw_random/virtio-rng.c] register_buffer() [drivers/char/hw_random/virtio-rng.c] sg_init_one() [lib/scatterlist.c] sg_set_buf() [include/linux/scatterlist.h] In turn, the sg_set_buf() function, when built with CONFIG_DEBUG_SG, actively enforces (justifiedly) that the buffer used within the scatter-gather list live in physically contiguous memory: BUG_ON(!virt_addr_valid(buf)); The combination of the above two facts means that whatever calls virtio_read() -- via the hwrng.read() method -- has to allocate the recipient buffer in physically contiguous memory. Although this ends up being a generic interface restriction that is not documented at the abstract hwrng level ("include/linux/hw_random.h", "Documentation/hw_random.txt"), the virtio-rng provider has not been changed to implement bounce buffering. Instead, existing core commits have accommodated the silent restriction, such as: - f7f154f1246c hw_random: make buffer usable in scatterlist. which would allocate "rng_buffer" with kmalloc(), and - be4000bc4644 hwrng: create filler thread which would allocate the new "rng_fillbuf" similarly. One call site remains that breaks the silent restriction: the add_early_randomness() function passes an on-stack array to hwrng.read(), via rng_get_data(), resulting in the following (valid) BUG, when CONFIG_DEBUG_SG is enabled: > ------------[ cut here ]------------ > kernel BUG at ./include/linux/scatterlist.h:140! > invalid opcode: 0000 [#1] SMP > Modules linked in: virtio_pci(+) virtio_mmio virtio_input virtio_balloon > virtio_scsi nd_pmem nd_btt virtio_net virtio_console virtio_rng > virtio_blk virtio_ring virtio nfit crc32_generic crct10dif_pclmul > crc32c_intel crc32_pclmul > CPU: 0 PID: 1 Comm: init Not tainted 4.9.0-0.rc0.git6.2.fc26.x86_64 #1 > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.fc26 > 04/01/2014 > task: ffff91f29de53240 task.stack: ffffb820000cc000 > RIP: 0010:[<ffffffff8347e3fc>] [<ffffffff8347e3fc>] > sg_init_one+0x8c/0xa0 > RSP: 0018:ffffb820000cf7d0 EFLAGS: 00010246 > RAX: 0000000000000000 RBX: ffffb820000cf858 RCX: 0000000000000028 > RDX: 0000262d800cf858 RSI: 0000000000000026 RDI: ffffb820800cf858 > RBP: ffffb820000cf7e8 R08: 000000000000006a R09: ffffb820000cf7f8 > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000010 > R13: ffffb820000cf7f8 R14: 0000000000000010 R15: 0000000000000000 > FS: 00007fffd6e6e140(0000) GS:ffff91f29ee00000(0000) > knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 00007fc67e24e000 CR3: 000000001bdad000 CR4: 00000000000406f0 > Stack: > ffff91f29be3b400 0000000000000001 ffffb820000cf858 ffffb820000cf848 > ffffffffc0056226 0000000087654321 0000000000000002 0000000000000000 > 0000000000000000 0000000000000000 000000002a14e409 ffff91f29be3b400 > Call Trace: > [<ffffffffc0056226>] virtio_read+0xc6/0x110 [virtio_rng] > [<ffffffff835be9ee>] add_early_randomness+0x5e/0xd0 > [<ffffffff835beaa5>] set_current_rng+0x45/0x160 > [<ffffffff835bee47>] hwrng_register+0xf7/0x130 > [<ffffffffc0056149>] virtrng_scan+0x19/0x30 [virtio_rng] > [<ffffffffc00467a8>] virtio_dev_probe+0x198/0x1e0 [virtio] > [<ffffffff835ebd53>] driver_probe_device+0x223/0x430 > [<ffffffff835ec0dc>] __device_attach_driver+0x8c/0x100 > [<ffffffff835ec050>] ? __driver_attach+0xf0/0xf0 > [<ffffffff835e972a>] bus_for_each_drv+0x6a/0xb0 > [<ffffffff835eb9c2>] __device_attach+0xe2/0x160 > [<ffffffff835ec193>] device_initial_probe+0x13/0x20 > [<ffffffff835eab93>] bus_probe_device+0xa3/0xb0 > [<ffffffff835e85f2>] device_add+0x382/0x650 > [<ffffffffc00929b0>] ? vp_modern_find_vqs+0x70/0x70 [virtio_pci] > [<ffffffffc00929b0>] ? vp_modern_find_vqs+0x70/0x70 [virtio_pci] > [<ffffffff835e88da>] device_register+0x1a/0x20 > [<ffffffffc00463f9>] register_virtio_device+0xb9/0x100 [virtio] > [<ffffffffc0093673>] virtio_pci_probe+0xc3/0x140 [virtio_pci] > [<ffffffff834c97b5>] local_pci_probe+0x45/0xa0 > [<ffffffff834ca81a>] ? pci_match_device+0xca/0x110 > [<ffffffff834cac33>] pci_device_probe+0x103/0x150 > [<ffffffff835ebd53>] driver_probe_device+0x223/0x430 > [<ffffffff835ec043>] __driver_attach+0xe3/0xf0 > [<ffffffff835ebf60>] ? driver_probe_device+0x430/0x430 > [<ffffffff835e9653>] bus_for_each_dev+0x73/0xc0 > [<ffffffff835eb47e>] driver_attach+0x1e/0x20 > [<ffffffff835eaea3>] bus_add_driver+0x173/0x270 > [<ffffffffc0099000>] ? 0xffffffffc0099000 > [<ffffffff835ecca0>] driver_register+0x60/0xe0 > [<ffffffffc0099000>] ? 0xffffffffc0099000 > [<ffffffff834c90d0>] __pci_register_driver+0x60/0x70 > [<ffffffffc009901e>] virtio_pci_driver_init+0x1e/0x1000 [virtio_pci] > [<ffffffff83002190>] do_one_initcall+0x50/0x180 > [<ffffffff83130ac5>] ? rcu_read_lock_sched_held+0x45/0x80 > [<ffffffff83275517>] ? kmem_cache_alloc_trace+0x277/0x2d0 > [<ffffffff831fa457>] ? do_init_module+0x27/0x1f1 > [<ffffffff831fa48f>] do_init_module+0x5f/0x1f1 > [<ffffffff8315df91>] load_module+0x2401/0x2b40 > [<ffffffff8315a7c0>] ? __symbol_put+0x70/0x70 > [<ffffffff830ec480>] ? sched_clock_cpu+0x90/0xc0 > [<ffffffff8323a9f3>] ? __might_fault+0x43/0xa0 > [<ffffffff8315e86b>] SYSC_init_module+0x19b/0x1c0 > [<ffffffff8315e9ae>] SyS_init_module+0xe/0x10 > [<ffffffff83909941>] entry_SYSCALL_64_fastpath+0x1f/0xc2 > Code: ca 75 2c 49 8b 55 08 f6 c2 01 75 25 83 e2 03 81 e3 ff 0f 00 00 45 > 89 65 14 48 09 d0 41 89 5d 10 49 89 45 08 5b 41 5c 41 5d 5d c3 <0f> 0b > 0f 0b 0f 0b 0f 0b 48 8b 15 05 ec 98 00 eb a3 0f 1f 00 55 > RIP [<ffffffff8347e3fc>] sg_init_one+0x8c/0xa0 > RSP <ffffb820000cf7d0> > ---[ end trace 8120a17353b469c4 ]--- Prevent this by allocating a temporary buffer in add_early_randomness() with kmalloc(). (The function add_early_randomness() should be called very infrequently, therefore it makes sense to trade speed for storage; i.e., to allocate the buffer only temporarily, for every call separately.) Cc: <stable@vger.kernel.org> # For v3.15+ Cc: Amit Shah <amit.shah@redhat.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Herbert Xu <herbert@gondor.apana.org.au> Cc: Kees Cook <keescook@chromium.org> Cc: Matt Mackall <mpm@selenic.com> Cc: Richard W.M. Jones <rjones@redhat.com> Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1383451 Fixes: d9e797261933 ("hwrng: add randomness to system from rng sources") See-also: 5e59d9a1aed2 ("virtio_console: Stop doing DMA on the stack") Reported-by: Richard W.M. Jones <rjones@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Signed-off-by: Laszlo Ersek <lersek@redhat.com> --- Notes: - (GFP_NOWAIT | __GFP_NOWARN) could be overly cautious, but I'm better safe than sorry. - If / when responding, please keep me addressed personally; I'm not subscribed to either linux-crypto or linux-kernel. Thanks. drivers/char/hw_random/core.c | 28 ++++++++++++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-) -- 2.9.2 -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html