linux-kernel - Re: [PATCH 1/2] mm/madvise: introduce PR_MADV_SELF flag to process

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Message-ID: <c44d373d-d72b-4e62-a613-a746a2c290e7@lucifer.local>
Date: Mon, 23 Sep 2024 20:34:35 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Suren Baghdasaryan <surenb@...gle.com>, Arnd Bergmann <arnd@...db.de>,
        linux-api@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH 1/2] mm/madvise: introduce PR_MADV_SELF flag to
 process_madvise()

On Mon, Sep 23, 2024 at 11:56:06AM GMT, Shakeel Butt wrote:
> On Mon, Sep 23, 2024 at 05:03:56PM GMT, Lorenzo Stoakes wrote:
> [...]
> >  SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >  		size_t, vlen, int, behavior, unsigned int, flags)
> >  {
> > @@ -1486,10 +1509,9 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >  	struct iov_iter iter;
> >  	struct task_struct *task;
> >  	struct mm_struct *mm;
> > -	size_t total_len;
> >  	unsigned int f_flags;
> >
> > -	if (flags != 0) {
> > +	if (flags & ~PR_MADV_SELF) {
> >  		ret = -EINVAL;
> >  		goto out;
> >  	}
> > @@ -1498,13 +1520,26 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> >  	if (ret < 0)
> >  		goto out;
> >
> > +	/*
> > +	 * Perform an madvise operation on the current process. No restrictions
> > +	 * need be applied, nor do we need to pin the task or mm_struct.
> > +	 */
> > +	if (flags & PR_MADV_SELF) {
> > +		ret = vector_madvise(current->mm, &iter, behavior);
> > +		goto free_iov;
> > +	}
> > +
> >  	task = pidfd_get_task(pidfd, &f_flags);
> >  	if (IS_ERR(task)) {
> >  		ret = PTR_ERR(task);
> >  		goto free_iov;
> >  	}
> >
> > -	if (!process_madvise_behavior_valid(behavior)) {
> > +	/*
> > +	 * We need only perform this check if we are attempting to manipulate a
> > +	 * remote process's address space.
> > +	 */
> > +	if (mm != current->mm && !process_madvise_remote_valid(behavior)) {
>
> Move the above check after mm is initialized i.e. mm = mm_access().
>
> Shakeel

Ugh, sorry silly one there! Reflexively put that check in the original position.

Enclose a quick fix-patch for it, will fix on any respin also.

----8<----
>From dc09e0edf1cf71a89cc4cfc3ec73fdae3c2ab86c Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Mon, 23 Sep 2024 20:33:07 +0100
Subject: [PATCH] mm/madvise: retrieve mm before checking

---
 mm/madvise.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 549b36d1463c..49d12f98b677 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1535,20 +1535,20 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
 		goto free_iov;
 	}

+	/* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */
+	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
+	if (IS_ERR_OR_NULL(mm)) {
+		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
+		goto release_task;
+	}
+
 	/*
 	 * We need only perform this check if we are attempting to manipulate a
 	 * remote process's address space.
 	 */
 	if (mm != current->mm && !process_madvise_remote_valid(behavior)) {
 		ret = -EINVAL;
-		goto release_task;
-	}
-
-	/* Require PTRACE_MODE_READ to avoid leaking ASLR metadata. */
-	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
-	if (IS_ERR_OR_NULL(mm)) {
-		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
-		goto release_task;
+		goto release_mm;
 	}

 	/*
--
2.46.0