Re: [PATCH 3/3] PCI: Prevent overflow in proc_bus_pci_write()

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Wed, 17 Dec 2025, duziming wrote:
> 在 2025/12/16 18:57, Ilpo Järvinen 写道:
> > On Tue, 16 Dec 2025, Ziming Du wrote:
> > 
> > > When the value of ppos over the INT_MAX, the pos will be
> > is over
> > 
> > > set a negtive value which will be pass to get_user() or
> > set to a negative value which will be passed
> > 
> > > pci_user_write_config_dword(). And unexpected behavior
> > Please start the sentence with something else than And.
> > 
> > Hmm, the lines look rather short too, can you please reflow the changelog
> > paragraphs to 75 characters.
> 
> Thanks for the review. I'll reflow the changelog to 75-character lines and
> avoid
> 
> starting sentences with 'And' in the next revision.
> 
> > > such as a softlock happens:
> > > 
> > >   watchdog: BUG: soft lockup - CPU#0 stuck for 130s! [syz.3.109:3444]
> > >   Modules linked in:
> > >   CPU: 0 PID: 3444 Comm: syz.3.109 Not tainted 6.6.0+ #33
> > >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> > > rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> > >   RIP: 0010:_raw_spin_unlock_irq+0x17/0x30
> > >   Code: cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e
> > > fa 0f 1f 44 00 00 e8 52 12 00 00 90 fb 65 ff 0d b1 a1 86 6d <74> 05 e9 42
> > > 52 00 00 0f 1f 44 00 00 c3 cc cc cc cc 0f 1f 84 00 00
> > >   RSP: 0018:ffff88816851fb50 EFLAGS: 00000246
> > >   RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff927daf9b
> > >   RDX: 0000000000000cfc RSI: 0000000000000046 RDI: ffffffff9a7c7400
> > >   RBP: 00000000818bb9dc R08: 0000000000000001 R09: ffffed102d0a3f59
> > >   R10: 0000000000000003 R11: 0000000000000000 R12: 0000000000000000
> > >   R13: ffff888102220000 R14: ffffffff926d3b10 R15: 00000000210bbb5f
> > >   FS:  00007ff2d4e56640(0000) GS:ffff8881f5c00000(0000)
> > > knlGS:0000000000000000
> > >   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >   CR2: 00000000210bbb5b CR3: 0000000147374002 CR4: 0000000000772ef0
> > >   PKRU: 00000000
> > >   Call Trace:
> > >    <TASK>
> > >    pci_user_write_config_dword+0x126/0x1f0
> > >    ? __get_user_nocheck_8+0x20/0x20
> > >    proc_bus_pci_write+0x273/0x470
> > >    proc_reg_write+0x1b6/0x280
> > >    do_iter_write+0x48e/0x790
> > >    ? import_iovec+0x47/0x90
> > >    vfs_writev+0x125/0x4a0
> > >    ? futex_wake+0xed/0x500
> > >    ? __pfx_vfs_writev+0x10/0x10
> > >    ? userfaultfd_ioctl+0x131/0x1ae0
> > >    ? userfaultfd_ioctl+0x131/0x1ae0
> > >    ? do_futex+0x17e/0x220
> > >    ? __pfx_do_futex+0x10/0x10
> > >    ? __fget_files+0x193/0x2b0
> > >    __x64_sys_pwritev+0x1e2/0x2a0
> > >    ? __pfx___x64_sys_pwritev+0x10/0x10
> > >    do_syscall_64+0x59/0x110
> > >    entry_SYSCALL_64_after_hwframe+0x78/0xe2
> > Could you please trim the dump so it only contains things relevant to this
> > issue () (also check trimming in the other patches).
> Thanks for pointing that out, we'll make sure to only keep the relevant stacks
> in future patches.
> > > Fix this by use unsigned int for the pos.
> > > 
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
> > > Signed-off-by: Ziming Du <duziming2@huawei.com>
> > > ---
> > >   drivers/pci/proc.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
> > > index 9348a0fb8084..dbec1d4209c9 100644
> > > --- a/drivers/pci/proc.c
> > > +++ b/drivers/pci/proc.c
> > > @@ -113,7 +113,7 @@ static ssize_t proc_bus_pci_write(struct file *file,
> > > const char __user *buf,
> > >   {
> > >   	struct inode *ino = file_inode(file);
> > >   	struct pci_dev *dev = pde_data(ino);
> > > -	int pos = *ppos;
> > > +	unsigned int pos = *ppos;
> > >   	int size = dev->cfg_size;
> > >   	int cnt, ret;
> > So this still throws away some bits compared with the original ppos ?
> 
> The current approach may lose some precision compared to the original ppos,
> but a later check ensures  pos
> 
> remains valid—so any potential information loss is handled safely.

That's somewhat odd definition of "valid" if a big ppos results in 
a smaller number after the precision loss that is smaller than size. 

-- 
 i.