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Subject[PATCH] x86: uaccess: fix regression in unsafe_get_user
Date
From: Arthur Gautier <baloo@gandi.net>

When extracting an initramfs, a filename may be near an allocation boundary.
Should that happen, strncopy_from_user will invoke unsafe_get_user which
may cross the allocation boundary. Should that happen, unsafe_get_user will
trigger a page fault, and strncopy_from_user would then bailout to
byte_at_a_time behavior.

unsafe_get_user is unsafe by nature, and rely on pagefault to detect boundaries.
After 9da3f2b74054 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses")
it may no longer rely on pagefault as the new page fault handler would
trigger a BUG().

This commit allows unsafe_get_user to explicitly trigger pagefaults and
handle them directly with the error target label.

Kernel bug:
[ 0.965251] Unpacking initramfs...
[ 1.797025] BUG: pagefault on kernel address 0xffffae80c0c7e000 in non-whitelisted uaccess
[ 1.798992] BUG: unable to handle kernel paging request at ffffae80c0c7e000
[ 1.798992] #PF error: [normal kernel read fault]
[ 1.798992] PGD 68526067 P4D 68526067 PUD 68527067 PMD 67f0d067 PTE 0
[ 1.798992] Oops: 0000 [#1] SMP PTI
[ 1.798992] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.0.0-rc6+ #14
[ 1.798992] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 1.798992] RIP: 0010:strncpy_from_user+0x67/0xe0
[ 1.798992] Code: fe fe 48 39 ca 49 ba 80 80 80 80 80 80 80 80 48 0f 46 ca 31 c0 45 31 db 49 89 c8 4c 89 c1 48 29 c1 48 83 f9 07 76 49 44 89 d9 <4c> 8b 0c 06 85 c9 75 3e 49 8d 0c 19 4c 89 0c 07 49 f7 d1 4c 21 c9
[ 1.798992] RSP: 0000:ffffae80c031fc40 EFLAGS: 00050216
[ 1.798992] RAX: 0000000000000040 RBX: fefefefefefefeff RCX: 0000000000000000
[ 1.798992] RDX: 0000000000000fe0 RSI: ffffae80c0c7dfba RDI: ffff8b3d27cce020
[ 1.798992] RBP: 00000000ffffff9c R08: 0000000000000fe0 R09: caccd29190978b86
[ 1.798992] R10: 8080808080808080 R11: 0000000000000000 R12: ffffae80c0c7dfba
[ 1.798992] R13: ffffae80c031fd00 R14: 0000000000000000 R15: 0000000000000000
[ 1.798992] FS: 0000000000000000(0000) GS:ffff8b3d28a00000(0000) knlGS:0000000000000000
[ 1.798992] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1.798992] CR2: ffffae80c0c7e000 CR3: 000000003a60e001 CR4: 0000000000360eb0
[ 1.798992] Call Trace:
[ 1.798992] getname_flags+0x69/0x187
[ 1.798992] user_path_at_empty+0x1e/0x41
[ 1.798992] vfs_statx+0x70/0xcc
[ 1.798992] clean_path+0x41/0xa2
[ 1.798992] ? parse_header+0x40/0x10a
[ 1.798992] do_name+0x78/0x2b5
[ 1.798992] write_buffer+0x27/0x37
[ 1.798992] flush_buffer+0x34/0x8b
[ 1.798992] ? md_run_setup+0x8a/0x8a
[ 1.798992] unlz4+0x20b/0x27c
[ 1.798992] ? write_buffer+0x37/0x37
[ 1.798992] ? decompress_method+0x80/0x80
[ 1.798992] unpack_to_rootfs+0x17a/0x2b7
[ 1.798992] ? md_run_setup+0x8a/0x8a
[ 1.798992] ? clean_rootfs+0x159/0x159
[ 1.798992] populate_rootfs+0x5d/0x105
[ 1.798992] do_one_initcall+0x86/0x169
[ 1.798992] ? do_early_param+0x8e/0x8e
[ 1.798992] kernel_init_freeable+0x16a/0x1f4
[ 1.798992] ? rest_init+0xaa/0xaa
[ 1.798992] kernel_init+0xa/0xfa
[ 1.798992] ret_from_fork+0x35/0x40

You may reproduce the issue with the following initrd:
truncate -s 8388313 a
SECONDFILENAME=bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
truncate -s 10 $SECONDFILENAME
echo "a\n$SECONDFILENAME" | cpio -o --format=newc | lz4 -l > initrd.img.lz4

This places the second filename in the cpio near the allocation boundary made
by lz4 decompression and should trigger the bug.

Fixes: 9da3f2b74054 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses")

Cc: Jann Horn <jannh@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Pascal Bouchareine <pascal@gandi.net>
Signed-off-by: Arthur Gautier <baloo@gandi.net>
---
arch/x86/include/asm/uaccess.h | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 780f2b42c8efe..2c272dc43e05a 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -724,7 +724,9 @@ static __must_check inline bool user_access_begin(const void __user *ptr, size_t
do { \
int __gu_err; \
__inttype(*(ptr)) __gu_val; \
+ current->kernel_uaccess_faults_ok++; \
__get_user_size(__gu_val, (ptr), sizeof(*(ptr)), __gu_err, -EFAULT); \
+ current->kernel_uaccess_faults_ok--; \
(x) = (__force __typeof__(*(ptr)))__gu_val; \
if (unlikely(__gu_err)) goto err_label; \
} while (0)
--
2.20.1
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