fs/proc: fix softlockup in __read_vmcore (part 2)

JIRA: https://issues.redhat.com/browse/RHEL-79112
CVE: CVE-2025-21694

Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

commit cbc5dde
Author: Rik van Riel <[email protected]>
Date:   Fri Jan 10 10:28:21 2025 -0500

    fs/proc: fix softlockup in __read_vmcore (part 2)

    Since commit 5cbcb62 ("fs/proc: fix softlockup in __read_vmcore") the
    number of softlockups in __read_vmcore at kdump time have gone down, but
    they still happen sometimes.

    In a memory constrained environment like the kdump image, a softlockup is
    not just a harmless message, but it can interfere with things like RCU
    freeing memory, causing the crashdump to get stuck.

    The second loop in __read_vmcore has a lot more opportunities for natural
    sleep points, like scheduling out while waiting for a data write to
    happen, but apparently that is not always enough.

    Add a cond_resched() to the second loop in __read_vmcore to (hopefully)
    get rid of the softlockups.

    Link: https://lkml.kernel.org/r/20250110102821.2a37581b@fangorn
    Fixes: 5cbcb62 ("fs/proc: fix softlockup in __read_vmcore")
    Signed-off-by: Rik van Riel <[email protected]>
    Reported-by: Breno Leitao <[email protected]>
    Cc: Baoquan He <[email protected]>
    Cc: Dave Young <[email protected]>
    Cc: Vivek Goyal <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

Signed-off-by: Baoquan He <[email protected]>

Author: Baoquan He

fs/proc/vmcore.c

@@ -405,6 +405,8 @@ static ssize_t __read_vmcore(struct iov_iter *iter, loff_t *fpos)
 			if (!iov_iter_count(iter))
 				return acc;
 		}
+
+		cond_resched();
 	}
 
 	return acc;