Re: [PATCH v2 2/3] PCI: Prevent overflow in proc_bus_pci_write()

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Dec 31, 2025 at 11:31:47AM +0200, Ilpo Järvinen wrote:
> On Tue, 30 Dec 2025, duziming wrote:
> > 在 2025/12/30 2:07, Bjorn Helgaas 写道:
> > > [+cc Krzysztof; I thought we looked at this long ago?]
> > > 
> > > On Wed, Dec 24, 2025 at 05:27:18PM +0800, Ziming Du wrote:
> > > > From: Yongqiang Liu <liuyongqiang13@huawei.com>
> > > > 
> > > > When the value of ppos over the INT_MAX, the pos is over set to a negtive
> > > > value which will be passed to get_user() or pci_user_write_config_dword().
> > > > Unexpected behavior such as a softlock will happen as follows:
> > > s/negtive/negative/
> > > s/softlock/soft lockup/ to match message below
> > Thanks for pointing out the ambiguous parts.
> > > s/ppos/pos/ (or fix this to refer to "*ppos", which I think is what
> > > you're referring to)
> > > 
> > > I guess the point is that proc_bus_pci_write() takes a "loff_t *ppos",
> > > loff_t is a signed type, and negative read/write offsets are invalid.
> > 
> > Actually, the *loff_t *ppos *passed in is not a negative value. The root cause
> > of the issue
> > 
> > lies in the cast *int* *pos = *ppos*. When the value of **ppos* over the
> > INT_MAX, the pos is over set
> > 
> > to a negative value. This negative *pos* then propagates through subsequent
> > logic, leading to the observed errors.
> > 
> > > If this is easily reproducible with "dd" or similar, could maybe
> > > include a sample command line?
> > 
> > We reproduced the issue using the following POC:
> > 
> >     #include <stdio.h>
> > 
> >     #include <string.h>
> >     #include <unistd.h>
> >     #include <fcntl.h>
> >     #include <sys/uio.h>
> > 
> >     int main() {
> >     int fd = open("/proc/bus/pci/00/02.0", O_RDWR);
> >     if (fd < 0) {
> >         perror("open failed");
> >         return 1;
> >     }
> >     char data[] = "926b7719201054f37a1d9d391e862c";
> >     off_t offset = 0x80800001;
> >     struct iovec iov = {
> >         .iov_base = data,
> >         .iov_len = 0xf
> >     };
> >     pwritev(fd, &iov, 1, offset);
> >     return 0;
> > }
> > 
> > > >   watchdog: BUG: soft lockup - CPU#0 stuck for 130s! [syz.3.109:3444]
> > > >   RIP: 0010:_raw_spin_unlock_irq+0x17/0x30
> > > >   Call Trace:
> > > >    <TASK>
> > > >    pci_user_write_config_dword+0x126/0x1f0
> > > >    proc_bus_pci_write+0x273/0x470
> > > >    proc_reg_write+0x1b6/0x280
> > > >    do_iter_write+0x48e/0x790
> > > >    vfs_writev+0x125/0x4a0
> > > >    __x64_sys_pwritev+0x1e2/0x2a0
> > > >    do_syscall_64+0x59/0x110
> > > >    entry_SYSCALL_64_after_hwframe+0x78/0xe2
> > > > 
> > > > Fix this by add check 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..200d42feafd8 100644
> > > > --- a/drivers/pci/proc.c
> > > > +++ b/drivers/pci/proc.c
> > > > @@ -121,7 +121,7 @@ static ssize_t proc_bus_pci_write(struct file *file,
> > > > const char __user *buf,
> > > >   	if (ret)
> > > >   		return ret;
> > > >   -	if (pos >= size)
> > > > +	if (pos >= size || pos < 0)
> > > >   		return 0;
> > > I see a few similar cases that look like this; maybe we should do the
> > > same?
> > > 
> > >    if (pos < 0)
> > >      return -EINVAL;
> > > 
> > > Looks like proc_bus_pci_read() has the same issue?
> > 
> > proc_bus_pci_read() may also trigger similar issue as mentioned by Ilpo
> > Järvinen in
> > 
> > https://lore.kernel.org/linux-pci/e5a91378-4a41-32fb-00c6-2810084581bd@linux.intel.com/
> > 
> > However, it does not result in an overflow to a negative number.
> 
> Why does the cast has to happen first here?
> 
> This would ensure _correctness_ without any false alignment issues for 
> large numbers:
> 
> 	int pos;
> 	int size = dev->cfg_size;
> 
> 	...
> 	if (*ppos > INT_MAX)

Isn't *ppos a signed quantity?  If so, wouldn't we want to check for
"*ppos < 0"?