** Also affects: linux (Ubuntu Eoan)
Importance: Undecided
Status: New
** Also affects: linux (Ubuntu Disco)
Importance: Undecided
Status: New
** Changed in: linux (Ubuntu)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Disco)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Eoan)
Importance: Undecided => Medium
** Changed in: linux (Ubuntu Disco)
Status: New => In Progress
** Changed in: linux (Ubuntu Eoan)
Status: New => In Progress
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https://bugs.launchpad.net/bugs/1860041
Title:
shiftfs: prevent lower dentries from going negative during unlink
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Disco:
In Progress
Status in linux source package in Eoan:
In Progress
Bug description:
SRU Justification
Impact: All non-special files (For shiftfs this only includes fifos
and - for this case - unix sockets - since we don't allow character
and block devices to be created.) go through shiftfs_open() and have
their dentry pinned through this codepath preventing it from going
negative. But fifos don't use the shiftfs fops but rather use the
pipefifo_fops which means they do not go through shiftfs_open() and
thus don't have their dentry pinned that way. Thus, the lower dentries
for such files can go negative on unlink causing segfaults. The
following C program can be used to reproduce the crash:
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
struct stat stat;
unlink("./bbb");
int ret = mknod("./bbb", S_IFIFO|0666, 0);
if (ret < 0)
exit(1);
int fd = open("./bbb", O_RDWR);
if (fd < 0)
exit(2);
if (unlink("./bbb"))
exit(4);
fstat(fd, &stat);
return 0;
}
Fix: Similar to ecryptfs we need to dget() the lower dentry before
calling vfs_unlink() on it and dput() it afterwards.
Regression Potential: Limited to shiftfs.
Test Case: Compiled a kernel with the fix and used the reproducer
above to verify that the kernel cannot be crashed anymore.
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