[PATCH 4.9 100/175] PCI: Return ~0 data on pciconfig_read() CAP_SYS_ADMIN failure

[Greg Kroah-Hartman]

From: Krzysztof Wilczyński <kw@xxxxxxxxx>

commit a8bd29bd49c4156ea0ec5a97812333e2aeef44e7 upstream.

The pciconfig_read() syscall reads PCI configuration space using
hardware-dependent config accessors.

If the read fails on PCI, most accessors don't return an error; they
pretend the read was successful and got ~0 data from the device, so the
syscall returns success with ~0 data in the buffer.

When the accessor does return an error, pciconfig_read() normally fills the
user's buffer with ~0 and returns an error in errno.  But after
e4585da22ad0 ("pci syscall.c: Switch to refcounting API"), we don't fill
the buffer with ~0 for the EPERM "user lacks CAP_SYS_ADMIN" error.

Userspace may rely on the ~0 data to detect errors, but after e4585da22ad0,
that would not detect CAP_SYS_ADMIN errors.

Restore the original behaviour of filling the buffer with ~0 when the
CAP_SYS_ADMIN check fails.

[bhelgaas: commit log, fold in Nathan's fix
https://lore.kernel.org/r/20210803200836.500658-1-nathan@xxxxxxxxxx]
Fixes: e4585da22ad0 ("pci syscall.c: Switch to refcounting API")
Link: https://lore.kernel.org/r/20210729233755.1509616-1-kw@xxxxxxxxx
Signed-off-by: Krzysztof Wilczyński <kw@xxxxxxxxx>
Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/pci/syscall.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/pci/syscall.c
+++ b/drivers/pci/syscall.c
@@ -23,8 +23,10 @@ SYSCALL_DEFINE5(pciconfig_read, unsigned
 	long err;
 	int cfg_ret;
 
+	err = -EPERM;
+	dev = NULL;
 	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
+		goto error;
 
 	err = -ENODEV;
 	dev = pci_get_bus_and_slot(bus, dfn);