Messages in this thread Patch in this message | | | From | baloo@gandi ... | Subject | [PATCH] x86: uaccess: fix regression in unsafe_get_user | Date | Fri, 15 Feb 2019 23:59:01 +0000 |
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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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