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From: Allen <allen.lkml@gmail.com>
Date: Tue, 30 Apr 2024 10:51:29 -0700
Message-ID: <CAOMdWS+jSC5UoO9nqYvRe_rRid_SOSuzK3c8M-NJ9-LKVhoCFg@mail.gmail.com>
Subject: Re: [RFC PATCH] fs/coredump: Enable dynamic configuration of max file
 note size
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Allen Pais <apais@linux.microsoft.com>, linux-fsdevel@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, viro@zeniv.linux.org.uk, 
	brauner@kernel.org, jack@suse.cz, ebiederm@xmission.com, 
	keescook@chromium.org, j.granados@samsung.com
Content-Type: text/plain; charset="UTF-8"

>  Will address it in v2.
>
> > If we're gonna do this, it makes sense to document the ELF note binary
> > limiations. Then, consider a defense too, what if a specially crafted
> > binary with a huge elf note are core dumped many times, what then?
> > Lifting to 4 MiB puts in a situation where abuse can lead to many silly
> > insane kvmalloc()s. Is that what we want? Why?
> >
>   You raise a good point. I need to see how we can safely handle this case.
>

Luis,

Here's a rough idea that caps the max allowable size for the note section.
I am using 16MB as the max value.

--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -56,10 +56,14 @@
 static bool dump_vma_snapshot(struct coredump_params *cprm);
 static void free_vma_snapshot(struct coredump_params *cprm);

+#define MAX_FILE_NOTE_SIZE (4*1024*1024)
+#define MAX_ALLOWED_NOTE_SIZE (16*1024*1024)
+
 static int core_uses_pid;
 static unsigned int core_pipe_limit;
 static char core_pattern[CORENAME_MAX_SIZE] = "core";
 static int core_name_size = CORENAME_MAX_SIZE;
+unsigned int core_file_note_size_max = MAX_FILE_NOTE_SIZE;

 struct core_name {
        char *corename;
@@ -1060,12 +1064,22 @@ static struct ctl_table coredump_sysctls[] = {
                .mode           = 0644,
                .proc_handler   = proc_dointvec,
        },
+       {
+               .procname       = "core_file_note_size_max",
+               .data           = &core_file_note_size_max,
+               .maxlen         = sizeof(unsigned int),
+               .mode           = 0644,
+               .proc_handler   = proc_core_file_note_size_max,
+       },
 };

+int proc_core_file_note_size_max(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos) {
+    int error = proc_douintvec(table, write, buffer, lenp, ppos);
+    if (write && (core_file_note_size_max < MAX_FILE_NOTE_SIZE
+                        || core_file_note_size_max > MAX_ALLOWED_NOTE_SIZE))
+.       /* Revert to default if out of bounds */
+        core_file_note_size_max = MAX_FILE_NOTE_SIZE;
+    return error;
+}

 Let me know what you think.

Thanks,
- Allen